I think it's more important to keep your roof clean than to worry about covering it. As Bingo says, abrasion is an issue. I've seen covered boats have their varnish and paint taken off after a winter under cover (improperly covered). Some covers are like sand paper after they get to be a few years old. The rubber/vinyl or whatever wears away and you're left with just strong, abrasive, threadbare corners wherever it touches anything.
Stay away from pine trees especially, and keep berries and fruity things off of it. Don't park under weak limbs either.
There is also the option to stock up on Margaritas and park it below 30 degrees N.
