My cookers have always been covered when not in use. Might go without a few days after a cook though.
I got into a conversation the other day with a buddy, he doesn't cover because "the airflow is super restricted, so any moisture that will get in has no chance to dry and turns to mold".
Neither of us have any scientific proof that either option is better, just thought I'd get a consensus from the brilliant minds here.
I think I'm gonna change my ways and only cover in the winter and storms. (except the griddle, that's stying covered)