It's not just the chemicals they use that you need to worry about pal, it's also the sponge that's been dropped on the ground, that's then rubbed over your paintwork (and everyone else's), followed by the dirty, gritty leather that'll be used to dry your pride and joy. You can bet they don't use a clean woolly mitt, 2 bucket method and a fresh, clean drying towel for each customer's car.
As well as telling the guy on the service desk that I didn't want my car washed, I also stuck a note on the dash in case the message didn't get through to the workshop. Seems to have worked. OCD? Maybe, but like you Mr GTD, I'd put a lot of time and effort into my car 2-3 weeks ago, claying, polishing, glazing and waxing, and the last thing I wanted was for the YTS guy undoing all the hard work.
Yes, I did take the 3 year service plan, so nothing to pay today! That was a nice feeling.