Even here in the Arizona sun, I don't cover mine. I tried with our last rig. I put my foot through a vent one time, and almost fell off on another. Like Andy and Julie said, it's a PITA. There were times we would have taken off for an overnight excursion, but it was too much effort to uncover and get rolling. I always felt putting it back on was too dangerous. There's a reason the factory personnel put on a safety harness when they are working on the roof. The injuries can be severe when you fall off a 10-12 foot FW.
The cover we had got caught in a wind storm when it was only a couple of weeks old, and was torn pretty bad. From then on, it was always flapping in the breeze. I threw it in the trash before it was a year old.
I wax my unit twice a year with Thetford RV wax which is a Carnauba wax with UV blockers. It's a full days work to wax it, but it sure looks nice when it's done.