Originally Posted by Sandra Dee

Detailers will tell you not to cover it if it is dirty beacause the cover can move the grit around and make microscratches in the paint. It is no worse though than washing the car incorrectly and making swirls in the paint.
Personally-I dust the car off with a California Duster before putting the cover on-which is what i do. The car is pretty clean and only has dust/pollen on it when i do this. If there are any other things-like bird poo-I get it off immediately since it is corrosive. Then i cover the car. Pretty simple and keeps the car looking good-guards against wandering cat prints thru the night.