The biggest enemy of keeping a car outdoors without a cover is not dirt, bird droppings, or even dogs weeing on your wheels, but the UV spectrum in daylight - esp. when the sun is shining.
Regardless of waxing, UV light degrades the paint and fades the colour of not just the exterior bodywork, but the interior as well! The dashboard and the seats of any car are also exceptionally prone to drying out and cracking where they heat up in sunlight.
Classic Additions or Covercraft make breathable covers suitable for outdoor use and for that reason can be put onto wet cars! Yes, the car will dry out under them, not rust.
I've got both my 964 and Boxster under Classic Additions lightweight outdoor covers, without any problems. Providing a cover fits correctly they should not rub on the car and it is possible, for an extra charge, to have a custom fit cover for your particular model.
It depends on how pedantic you want to be, but my usual tack is if the car's a bit dirty from use that day and I've not got time to wash it properly, a dousing with the hose is enough to knock anything really grotty off its well waxed/polished paintwork, before putting the cover on and leaving it to dry off underneath.
