I always clean my car before it goes into the dealer for servicing / other work, as I think its easier to see any dealer-inflicted damage on a clean car than it would be on a dirty one.
I accompany the service advisor when they inspect the car for any damage when I take it in, and get them to agree it's damage-free, hang my 'do not wash' sign in the car, instruct them not to wash it, get them to write this on the worksheet and tie a 'do not wash' tag on the keys.
When I go back to collect it, I inspect it unaccompanied before going into service reception, so if there was any damage that wasn't there when I took the car in, I could highlight it to them (thankfully, hasn't been any to date).