I 'm always playing with condition in green bar - not 99% but over 85%. In finals - semi finals 99% condition and morale.
If the game isn't going well - I 'm loosing or is going to a draw, I put hard attack, whole pitch pressure, hard tackling and maybe few red arrows. For doing this you must have good condition.
If a game is easy, it's a ok to have less condition but then you risk with injuries. 1 injury maybe cost you 15 red packs for healing.
Condition has nothing to do with grow/improve.