Hey Belier. It depends.
Training sessions aren't mandatory per say. But it
is a matter of priorities really.
Do you want to keep your Team bonuses at a certain point? Is the match not that much of a priority? Are you prioritizing player quality gains over team bonuses or vice versa? Maybe you currently don't have the condition to spare, or maybe you do - whether or not, you can chose some easy drills over hard ones and train your whole team in bulk just to maintain your team bonuses.
As you can see there is a lot of decision making behind the curtains.
It's definitely up to you as a Manager to decide on what to do and to think of a strategy for your team and the Club as a whole.