Lots of people do it here. If you care about your customers and want to minimize damage they can do, get a grit trap for the bucket. I recommend signage that says "Only for windshields and grills" and/or "Rinse brush thoroughly before using" and/or "Not responsible for scratches." Most just use a nylon foam brush or scrubber brush on a pole. They will get stolen, but it isn't usually a big problem. If I had something like that out at my wash, it'd be gone in an hour. Some homeless people stole the lids off my dumpster last night.