The time of year you go shouldn't make much of a difference in re driving yourself. Disney traffic is never as bad even during the most crowded times as a medium-sized city would be.
If we have a car, I usually try to drive to all the parks but MK. Driving to MK involves not only waiting for the tram, but also having to ride the monorail or ferry boat to get to the park itself. Busing from the resort is almost always quicker to MK, since the buses drop you off directly in front of MK.
As far as a crowded parking lot, I've been when the parks were packed near to capacity closing, and there was still plenty of space left in the parking lot. Of course the later you get the farther away from the park you get, but the tram eliminates the need to walk any farther than to the tram path. I haven't noticed that the wait for the trams is any different when you get there on a crowded day than if you get there early or on a low crowd day. My only real complaint about the trams (these days) is the unwillingness of fellow guests to heed the announcement of "No further boarding at this time ..." and they come on anyway, not realizing there's another tram right behind that one. I don't think I've ridden a tram recently where that announcement hasn't had to be said at least 4 times before people finally stop trying to board.
Driving to the non-MK parks however, in my experience, is almost always faster than waiting on a bus.
