Depends where you live...lots of dust? Lots of wind? Lots of moisture? Some claim they cause more damage by trapping moisture and humidity. Some say if in an area with lots of wind that the cover is flapping in the wind part way threw winter and and constant rubbing causes more damage. This topic gets alot of discussion every year right about this time of year.
Personally I have never covered. Give the unit a good waxing in early fall to take it through he winter. If you live far enough north like I do, the UV rays really are not a factor over the winter.
2011 GMC Sierra 2500HD
2014 Rockwood 8289WS, Diamond Pkg.