I'm just wondering how many people ask their dealers NOT to wash their cars.
I guess this doesn't bother most people. 
I think it might bother quite a few of the forum members who'll respond to your post; especially those who are regular visitors to the Detailing section of the forum.
Do I let them wash my car? Definitely not, especially if I've recently done a full detail, which takes me a couple of days. I don't want some sponge-monkey at the dealers undoing all of my hard work.
The service advisor I usually see at my local dealers is into car detailing and he fully understands why I don't want them to wash my car with their single bucket of dirty water and grit-embedded sponge. He said with the exception of one of the guys prepares their new cars, he wouldn't let anyone at the dealership wash his car, so not a good endorsement of the abilities of the guy(s) who do the service washes.
I hang my 'do not wash' hangar from the rear view mirror, tie a 'do not wash' tag on my keys and get them to write the same on the work sheet. It's worked so far. So they're not tempted to wash it, I also try and make sure it's spotlessly clean when it goes in for service and any warranty work; not always possible though as if the weather's bad on the day I take it to the dealer, it gets dirty driving it there.
Also, I find it easier to check a clean car for any garage-inflicted damage after they've finished working on it.