Fair do's on not asking them to wash your car (gritty sponge, swirls, or even an automatic car wash), but asking them not to adjust the seat is a step too far IMO. Unless the person moving it around at the dealership between car park and workshop (and even test drive to ensure fault is fixed - assuming that it needs one, dependent on fault) is the exact height and build as you, i'd expect them to adjust your seat to something they are comfortable driving. Imagine the look on your face if some 5-foot-nothing technician has to sheepishly tell you they crashed your car in the car park because they couldn't reach the brake pedal, as they weren't allowed to move the seat?
Readjusting the seat to your preferences might take a minute - no great hardship, it's when they piss about with your radio station presets that I take exception to.