Why would you want to delay that?
ALL crew members receive the same XP after a battle. If your battle got you 400XP, each of 4 crew members gets +400XP, not +100. If your crew member needs 1800XP till next skill level, you need to earn just 1800XP, 9000.
That makes tanks with large crew better for training as you train more crew members at the same time as all of them get the same, full XP.
There are two exceptions to that rule.
- Elite Training gives double XP to the crew member with lowest XP at the moment of "upgrade".
- Crewmen who die during the battle only receive 50% of the exp awarded.
Let's say your crew is:
Gunner 1: 83800XP
Gunner 2: 84100XP
You're in an Elite vehicle with Elite Training enabled. You won a battle, earning 400XP. It was the first victory in that vehicle so you received the daily 2x bonus. It's also a vehicle on a special mission list, with mission bonus "triple experience for the crew". During the battle your driver and gunner got killed.
base 400 x2(first victory) = 800 x3(mission) = 2400
Commander: 84500XP + 2400 = 86900
Gunner: 84200XP + (2400/2(killed)) = 85400
Driver: 84600XP + (2400/2(killed)) = 85800
Gunner 1: 83800XP + (2400*2(elite)) = 88600
Gunner 2: 84100XP + 2400 = 86500
As you see Gunner 1 got 4800XP thanks to the Elite Training (and starting off with lowest XP in the crew) and it got him ahead of all the rest.
Usually, there is no reason to delay choosing a skill if you only know which one to choose. The golden [+] is of no use, while any of skills that grow with percentage grows by strides in the early stage.
You may consider delaying in two cases:
- If you choose one that works after achieving 100%, that's when you might consider delaying the choice - XP still gets collected, and you can change your mind about what to choose.
- If you plan to retrain your crew for silver to the new tank, losing 10% of the base skill. The XP points accumulated under the golden "+" immediately fills the base skill deficit; to recover your crew from 90 back to 100% you need 39,153 exp and if you collected that many under non-assigned skill, you'll be back to 100% immediately (the XP consumed, obviously).