I always request "do not wash" when I drop a car off for warranty work. I envision a couldn't-care-less teenager adding 2.5 million hairline scratches as he washes it with a rag and dries it with a towel he just picked up off the ground.
If you're comfortable with someone like this going at your car with a rotary buffer and who knows what product, sure, let the dealership do it. Personally, I'd pay a pro.