I used to. I worked at a dealership, and knew most of the mechanics well enough to buy them a beer after work, and hear stories. For the most part those guys were just there to do a job, it didn't matter to them what they were working on because they had seen the cars when new as well as various states of age, condition and or disrepair. We had an NSX or two come through our shop, as well as S2000s and other "fast cars" that came through our used dept (Corvettes, Z28s, WS6 Trans Ams, various Mustangs as well as some fast Germans ect). I never saw or heard of anything going on with customer cars.
I took my Integra GSR to a Honda shop in Moon Township near Pittsburgh. My Uncle was working sales there, and got me a family price on new rotors and brakes. I was visiting with my Uncle while he smoked a cigarette outside when I heard the distinctive sound of my GSR (header, cat back exhaust), wide open VTEC- I looked over to see my car bombing down Beer School Rd. at twice the speed limit.
I walked around the shop and saw the guy bringing the car back in- The culprit was the dealers car prep. "all washed up for you" he said
"And slightly wrung out?" was my reply, followed by what I had just seen him doing with my car on the road. His manager wasn't much older than he was, and I doubt my complaints went much further than passing through his ears, but I let him have it pretty good.
No harm done in this instance, but ever since that day I don't really trust to leave my cars while they are serviced. I prefer to wait.