If it is already wet don't bother covering it now, but I would not make a habit of leaving it uncovered. Always cover it when you are not riding (or put under shelter).i have a cover, but my bike was already wet. would it do any nice if i covered it? i mean, it's wet! if it's wet and covered isn't that worse?
plus, i am afraid of scratching it. i know i know, covers are made NOT to scratch the bike, but i am afraid anyway...