The parking fees are charged only for overnight stays so if you visit another resort you won't be charged under any circumstances. You may have an issue getting into the lot, depending on the resort, however.
The Magic Kingdom area resorts -- Contemporary, Polynesian, and Grand Floridian -- are tougher on parking, especially during busy times. The Contemporary especially will not allow cars into the lot unless you have a reservation at either the hotel or one of its restaurants. The Polynesian and Grand Floridian will sometimes use that policy as well, but not all the time. These measures are specifically to stop people from parking there all day, not that it completely stops that anyway.