Regarding a cover....for us, the negatives outweighed the positives. It is a real pain getting the thing on and off, for one. But the worst part is that it flaps in the breeze, even when pretty tightly secured, and can make wear marks on the rig. If not tightly secured it can come loose and cause a lot of problems in windy conditions, including flapping against the side of others rigs in storage (which doesn't make the neighbors happy at all). Indoor storage is obviously the best, but they are few and far between in my area.
We do use the tire covers when stored, or when camped for a significant period of time (or in direct intense sun). They are relatively easy to get on/off and aren't as prone to flapping around or loosening up.
- Mike