First for my still Little experience the risk of injured it is biger If you traine your player until they go to 50%, they also gaine less experience whend they passe to yellow. I have Being noticed that the same kind of training gives more experience If the player is more 80% than whend he already with 79% or less.
If you still have 5 hours to your next game you can gaine 5 to 10% of health, so you are waisting those 5 or 10% and also green packs. Even If the player play with 90 or 85% they still can do well.