You've asked a much-discussed question that has many varying opinions... Here's at least one recent thread that you can read through and see what people say... I'm sure there are more threads discussing this...try doing a search and see what you come up with. http://www.airforums.com/forums/f161...eam-34310.html
Many owners say it's not good to cover Aristreams because of the aluminum, clear coat, etc, while others say it's not a problem with newer cover systems, etc... Like I said...lots of opinions on this.
We have chosen not to cover ours with a cloth cover or canvas, but it is protected under a "Bambi Port" we built on the side our house...and we keep it washed and will wax it about every 6 months to keep it looking new and fresh. Older Airstreams may have difference needs, too, as the aluminum and clearcoat on older ASs are different than newer ones.
Good luck! What it really boils down to is what you're comfortable with and what makes sense to you...