Bibo1GerUk, on 18 June 2019 - 04:40 PM, said:
Just switch them on. We get so many nowadays, just use them. Think of them like Premium Time, fire and forget. If you think too much about them you'll put yourself under pressure to be good and then fail 
ATHFjman18, on 18 June 2019 - 02:47 PM, said:
Don't worry about crap like time of day, tank tier, win/loss ratio....Just set it and forget it!
I agree 
I simply load'em and have fun. Following the Law of Large Numbers, if you play a lot of games using boosts you're bound to have both ups and downs, compensating each other.
I prefer to use x4 and x5 XP boosts on the highest-tier tank I'm grinding, best if coupled with the x3 or x2 automatic boosts that come as rewards from the Op, for massive earnings (when the stars align you can get from 15 to 20 thousand xp points in one battle).
When grinding other tanks I use the x2 and x3 XP boosts. For instance last week I was grinding both the Standard B and the AMX M4 45, and I used the x5/x4XP boosts for the former and the x3/x2XP for the latter.
When I'm playing a Premium tank (or, sometime, even an Elite TT Tank) I try to use my CrewXP bonus, the x10 and x8 with crews in dire need of experience to reach Mastery or Sixth Sense as fast as possible, the x4 otherwise. Just be careful with untrained crews, you might have a hard time training one in a high-Tier tank while you might be just fine below Tier V. Just do what you're comfortable with.
If you think you're having a hard time pulling out consistent games in Multiplayer, or if a stock tank is particularly awful, consider a few boosted War Stories to get past the bumps
I usually use silver boosts on high earning premiums (even a CKV1 will do when top tier) when I'm not using CrewXP boosts.