As a customer, the sign would be a bit irritating to me. I think the whole tip system is a bit ludicrous. I don't know exactly how tips became a custom, but the fact it is becoming expected for virtually every service is just stupid. It's basically saying to me that if I don't tip, you are going to give me crappy service. As someone else mentioned, you should be pricing your service properly so I shouldn't have to tip you. I tip servers because I know they make squat for a wage. I'll tip more if I received exceptional service above and beyond just bringing me my food. So IMO, the only reason a detailer should earn a tip, is if he/she did something beyond the expected service. Why should I tip you if you did a good job on the polish/wax of my car since that is an implied requirement of the service? Now if you did something extra, like vacuumed the drivers side floor because of some sand, and the interior wasn't part of the service being done, then I can see a tip being called for. Or if I just feel like you did a lot of work, and undercharged me for what was done, a tip would be warranted. As was mentioned, people will give a tip regardless of a sign. I think it will just irritate those of us that feel all of the fair wage should be priced into the service.