XP management

So my curious questions of the day is this.
Do you promote your soldiers as soon as you have enough XP or do you bank it so that you can level to the next level -1? Do you find that it better to sit at a particular level to harvest XP or do you put your points into the next level so that you can become more successful in your fights and therefore gather copious amounts of XP to improve?
Do you promote your soldiers as soon as you have enough XP or do you bank it so that you can level to the next level -1? Do you find that it better to sit at a particular level to harvest XP or do you put your points into the next level so that you can become more successful in your fights and therefore gather copious amounts of XP to improve?