First of all, I want to point out the importance of having a good crew in this game. You will never see a good player driving his tier 9/10 tank on stock crew, or crew with only 1 skill. Crew skills are the little extra, that can benefit all aspects of tank performance. They can be used to compensate for the weak points and enhance the strong points of the vehicle. Specific examples will come below, as I give more detailed explanation about the skills.
I am not going to state here all the obvious information about skills/perks. You can find the most basic stuff in here: http://wiki.worldoftanks.com/Crew#Skills_and_Perks . Another very helpful item, which will allow you to configurate crew for the desired vehicle and see what the actual effect of certain skills is, as long with whole bunch of other useful stuff is Tank Inspector.http://tankinspector.sinaapp.com/ For the moment it is free to use and I strongly recomend it to everyone.
The following info concerns skill setup on different types of vehicles, usefulnues and utility of the skills/perks.
1. Repairs: This is a must-have on nearly every tank, especially for heavies and mediums. TDs with more active gameplay can also benefit from it. Not very useful on low tier scouts and all arties. SPGs have generally low HP, and are usually 1-2-shots kills. Same goes for scouts: if u get detracked or engine is knocked out, you use your repair kit, you will probably not have enough HP to survive another hit anyway. However, high tier LTs light AMX 13 90, 13 75, WZ131/132 and T71 can benefit from repairs but it still isnt their most essential skill.
2. Firefighting: Currently not very useful skill, especially with the introduction of premium caonsumables for credits. This skill should only be trained when all other useful skills/perks are already obtained. It is more useful on tanks with a lot of HP like high tier heavies and destroyers.
3. Camouflage: Very important skill for some types of vehicles. It also enables yout to disappear faster from enemy radar, after hiding yourself. Absolutely essential skill to scouts/light tanks, very very important for medium tanks and TDs. It has some limited use on heavies and high tier SPG, because of their naturally low camo factor and big size. Light tanks crew should train this as first crew. Some good players actually prefer this skill over repairs for thier tier 9/10 medium tanks. This is completely understandable, given the good camo factors of most medium tanks.
4. Brothers in arms: A very interesting perk. It has to be 100% on all crew members, in order to get active. Currently, the skill remains active even if a crew member is knocked out during battle. It enhances general crewmember profession AND all crew skills: this is why it enhances for example repairs, camouflage ... etc. This skill is useful on all type of tanks, including SPG. HOWEVER, my advice is to take other more important skills before getting BIA. I mostly recommend it for 3rd/4th skill.
Mentor: Usefulness of this one is limited. It is good if you plan to use a certain crew for a very long time, i.e. developing 4 or more skills per crewmember.
Recon: It is not very useful skill for most tanks, can be good on scouts. It is less effective than situational awareness skill! It should only be trained after getting all the better skills first.
Jack of all trades: Personally I dont think this skill is worth the grind. It is useful, but there are a lot better skills/perks to invest your XP in. For heavy tanks, however, this skill is pretty good, since they are the main targets of SPGs, leading to considerable crew/module damage.
Eagle eye: 99% useless perk! Do not get this one, unless it is your 6th skill (lol)
6th sense: Probably the best commander skill, maybe the best skill in the game. It is always useful to know when you are being spotted so that you can get in cover or start making evasive maneuvers. This should be the first park on almost any crew, excluding SPGs (they need BIA and camouflage more)
Armorer: Generally useless skill, cuz the gun is usually the hardest thing to get damaged. However, it can be useful on JPZ auf E-100, Borsig, SPGs, and other lightly armoured vehicles. But this is not a priority skill: there are more important stuff to get first.
Snap shot: Very useful skill for every type of vehicle - turreted or turretless! This is especailly useful for the following occasions:
b) all brawling tanks
c) all tanks with big reticle dispersion after gun/turret movement. Also applies to TDs
I prefer getting this skill on almost any tank I drive. However there are some tanks that have so little aim dispersion, that renders this skill almost useless. Check Tank Inspector to see what the dispersion on the certain vehicle is. For ex. I dont have this skill on my E-50M.
Deadeye: Pretty interesting stuff here. I find it useful on machines with high rate of fire, using AP or APCR shells. It is even more useful on crews with vents, BIA and some premium food. 3% in the wiki may not seem a lot, but I have the strange feeling that the real in-game value is more. However, it is not advisable to take this as your first perk.
Designated target: Perk description looks nice, but in reality it isnt that useful. It is worthless for SPGs. Personally I dont have this skill and will never train it on any crew.
Clutch braking: Useful skill on any vehicle that already has a good hull traverse. I feel the border line is about 35 degr/sec. Anything turning lower that that cant really benefit from clutch braking. This skill is very good on fast TDs, some light tanks and bat chat 25t. However it is also useless on some very rapidly turning vehicles - the borderline here is about 55 degr. - any more than that and you dont really need this skill.
Smooth ride: This is the ultimate driver skill in brawling tanks. You need this if you want to make quick pokeshots, peek-a-boo, working over ridgelines. Note, that the skill only works while moving forward and backwards and it is ineffective while turning. The driver should be cautious about that: best way to use this skill is when moving forward, popping out of cover (if needed), taking a quick shot at the enemy, then go back. It works great for peek a boo over ridges. The fact that this skill isnt effective means that its usability in SPGs, TDs and light tanks is somewhat limited. It can be used in light tanks, but one should be aware of it and shoot only when driving forward.
Off-road driving: Another great driver's skill. This also enhances hull traverse. It is especially useful on tanks with high ground resistance. Check Tank inspector to see if it is worth getting on your tank. I fint it mostly useful on my mediums.
Controlled impact: Ramming other tanks is fun but in most cases it is not a viable tactic, sothsi skill is useless. Completely worthless training this in a light tank!
Preventative maintenance: This perk is designed to be used on any tank with frontal engine/gearbox like E-75, Panthers, E-100, Maus, JPZE100, Obj 263 etc. It is pretty good, but should not be the first thing to train.
Signal boosting: very limited use. Good only on ELC AMX, KV-5, some other tanks with poor signal range.
Situational awareness: Probably the best skill here, being more effective than commander's recon skill. This should be trained on all light tanks and mediums as 2nd/3rd skill.
Relaying: Another useless skill. There are a lot more important stuff than this.
Call for vengeance: I find it generally useless on msot vehicles, but it is not such a bad idea to train it on some tanks... after camo, bia, repairs, situational awareness :)
Safe stowage: Probably the most useful skill here, absolutely essential to all tanks with frontally positioned ammo rack! This should be the first perk to have on your loader. Remember, you only need one loader with that perk.
Adrenaline rush: generally USELESS, as it becomes active only below 10% vehicle HP. Some players use it on Maus, but diong the math shows you that you'll have to have less than 300 hp to make benefit of this perk... meh...
Intuition: Pretty interesting stuff here, especially on vehicles with 2 loaders, since this perk is cumulative. However, I dont think it is very useful on autoloaded guns. I personally use it on KV-5, KV-4, T34, E-100. It is a nice optional skill.
Now, about building your crews. I believe there are some 'milestones' that you need to complete, while training your crew. First skill training on most vehicles can begin on tier 5, sometimes even on tier 6. By the time you get to tier 8, you should have at least one full (100%) skill and the seccond skill should be at least 70%. If you can afford it, you can train crew with gold. By the time you get to tier 10 you should have a good 3+ skill crew.
Also it is useful to know the difference between ''skills'' and ''perks''. Skills are effective at any given % of their development, while perks become active only after they reach 100%. This is the reason why it is not a bad idea to take some essential perks as first or 2nd skill, while they are still fast to develop and less XP is needed (for ex. Situational awareness, safe stowage, probably deadeye).
Basically two types of heavies here: classic heavies like IS-7 and medium-like heavies like T110E5 or 113. Training starts similar on all heavies: you take repairs for everyone. when reaching 100% repairs (prefferebly +40% on the 2nd skill) you reset your crew skills and take 6th sense on the commander and safe stowage on one loader. After that it is the matter of getting the skills you like the most. I advise that BIA is trained as 3rd or 4th skill. For medium-like heavies you can have clutch braking . If driving a german heavy tank, you need to get preventative maintanance as 3rd skill, maybe even 2nd.
Here is my setup on E-100
Commander: 6th sense, repairs, BIA, firefighting
Gunner: repairs, snap shot, BIA, firefighting - will change it for deadeye when 100%
Driver: repairs, smooth ride, BIA, preventative maintanance at 90%
Radioman: repairs, awareness, BIA, firefighting
Loader: safe stowage, intuition, BIA, repairs
Loader: BIA, intiution, repairs 40%
This is where it gets a bit complicated. There are many types of medium tanks and even more types of gameplay. I'll afford to break it down to 2 types: soviet mediums and all others. Soviet meds at high tier generally have very good camo, strong turret and low alpha dmg with high rate of fire. Their mobility is good. Other mediums like the batchat and leopard 1 have no armor, but pretty amazing guns, mobility and camo. There are also the M48 patton and the STB-1 which have amazing gun depression, but somewhat inaccurate guns that makes them more suitable for medium and close range combat. So, USSR meds can benefit from camo and deadeye more than other tanks, bat chat needs camo, smooth ride and probably clutch braking, and STB and patton can use awareness, snap shot and smooth ride.
There are a lot of good medium tank drivers, that prefer full camo crew over full repairs. Here are proposals of crews for T-62A an bat chat 25t.
Snap shot and smooth ride can be changed to camo for full stealth mode :)
+skills are optional 5th skills. Personally I would get BIA as fourth skill instead of 3rd, I would also prioritize my driver skills the following way: off road driving, smooth ride, camo, BIA + repairs.
BIA may be even used as 2nd skill here if the objective is to get maximum view range while still training your 3rd skill. The key of batchat builds is reaching high camo rating and view range. Finding a suitable balance of repairs/utility skills like snap shot and smooth ride is also important and some players prefer the repairs, but I generally stick to the "Do not get hit" idea and value utility skills over it(repairs).
First I have to say that not all light tanks are scouts and some medium tanks usually do the scouting when there is no light tank in the team. Most essential skill for light scouts is camo. Repairs is not imortant here.
Here is the built for Chaffee with 2 skills (3rd is optional):
Commander: 6th sense, Camo + BIA
Gunner: Camo, deadeye + BIA. Snap shot may be used instead of deadeye.
Loader: Camo, repairs + BIA. Safe stowage can be used instead of repairs.
With situational awareness as first skill, coupled with coated optics Chaffee gets pretty big view radius, which is important, given the fact that its top speed is mediocre. For 2nd skill on driver I personally would prefer off-road driving over clutch braking.
It is tricky here, depending on what tier arty u want to play. Low tiers up to tier 6 can actually benefit from Camouflage skill, whereas high tier arties can have camo skill later. I offer a build for high tier arty here. You start with camo skills for everyone except driver and gunner - they get snap shot and clutch braking. After reaching 100% and starting the 2nd skill, you reset them for BIA and start again.
Build looks like that:
Commander: BIA, camo, Jack at all trades + recon
Gunner: BIA, snap shot, camo + armorer
Driver: BIA, clutch braking, camo + off road driving
Radioman: BIA, camo, call for vengeance + awareness
Loader: BIA, camo, stowage + intuition
Loader: BIA, camo, intuition
Well, it is no rocket-science here. You need to start with CAMO for everyone, then change it on the commander for 6th sense. It is good to also have a high level of repair skill for those occasions whene you get detracked. Clutch braking is good vs medium tanks, and snap shot is a must for every turreted TD. I feel that BIA is pretty much optional here, since it really doesnt boost DPM that much. German waffentraegers can use armorer and jack at all trades, cause they suffer from heavy crew/module damage.