For me it's kind of a relief when the car gets dirty (say, if I get caught in a rainstorm on a long drive AND the wheels needed brake dust cleaning BEFORE the trip).
When it gets bad like that I just leave it, knowing it's got a good coat of wax on it under all that grime.
THAT means now that it's winter I can drive it more often and not be so concerned about getting it dirty.
I'll probably leave it dirty until there's a nice string of good weather predicted.