I'm not 100% sure, but believe win bonus helps to achieve better results.

Usually I will set the win bonus to maximum soon in a new season, and keep this on. Sometimes I may reduce the win bonus e.g. when league position is determined. Usually the game result is bad.

If you set win bonus to maximum, you may suffer financial losses league games. So you may consider carefully. But for cup and CL, I recommend to set bonus to high because the win award is usually far higher than your win bonus.

Good luck.