I specifically asked my HVAC guy about covering in winter. According to him it’s a big no-no (something about trapping moisture).
He said all you have to do is hose or blow out and accumulated leaves periodically. Says the units are built to live out of doors.
Everything I’ve read says that a full covering on all sides is bad, but that a cover over the top only can be beneficial since snow accumulating on the fan blades can cause them to become unbalanced. Even that seems like a bit of a stretch to me.