Jeck
You don't mention if the bike is stored inside or out. If outside I would definately want a cover that was made for the bike and waterproof. If inside, just a cheap cover to keep the dust off. One thing to look for is a cover that has a heat resistant material for that area where it will touch your pipes. When I get back from a ride I normally cover the bike right away and the pipes are still warm. Obviously if the cover does not have this you would need to go back out and cover it later.