"Clean up after yourself" or "most anything goes"? I have seen both models work. For me it's a personal decision. The people that have nicer, newer carwashes and more invested seem to be the one less tolerant of the the mess makers. Also it seems most operators that do their own cleanup are less tolerant.
As all costs rise, especially labor, muddy/messy business doesn't pencil out as well. My net (ex labor) on a self serve customer is about .39/minute, and that is higher than most. Labor costs for me are about .20/minute. Someone could easily get into my pocket by making a mess.
Most businesses charge a client based on the amount of involvement and their projected profitability. We don't have that luxury, so we either decide to put up with it, or run them off.