I've been covering my highly waxed BMWs for 5 years without a problem. Be sure you purchase a cover that breathes - so it doesn't trap moisture for a long time between it and the paint in the hot sunshine (summer).
Covers keep my cars clean and key-scratch free. Once you have the hang of it you can put it on and off in about one minute. Its especially nice to have a cover when you have to park in a crappy neighborhood, parking garage etc.