It just takes habitual reinforcement of the fundamental skill to clean, which shouldn't really detract from other teaching points. It's should be just like how we reinforce breathing skills every day even though some of them learned it 6-7 years ago.
Yes, the drum corps have lots of time to clean things, but I really don't think they're spending as much time as you think specifically cleaning 1st position angles. Does it get mentioned? Absolutely. Does anyone have a "1st position cleaning block?" Doubtful.