I use a nice fitted breathable cover, can't remember the brand but I could get the name if it interests you.
My advice is this, a cover is great if the car is inside a garage, it keeps the dust off and prevents people from bumping and scratching it. If the car is outside in the elements it depends you have the potential for wind movement which could cause paint damage or trapping moisture, but it's a trade off with the sun and other things so I guess it all depends, mine is inside so I keep it covered and I like the fitted cover, easy to put on and it stays right where you put it with no movement once it's on. I just wish it was protecting a fresh paint job instead of the 20 year old paint, some day soon.
Cory