No, but two things to watch out for:
(1) A loose cover flapping in the wind will rub the paint off your car. As it did for the guy who parked his Rabbit at my house one winter.
(2) When I was in Daytona for Bike Week, every bit of unpainted metal on my bike started to rust within two days of arriving by the ocean. Is it possible that slapping a cover over your car in San Francisco will have the same effect? (I know that the Hot Rod Magazine tips for storing a car mentioned this possibility).