I know stew, its that watch them like a hawk to make sure that any marks I can prove they caused vs I don't think I can watch this!
Anyway, I've made sure they know how much they cost and that they are fully insured and can do 19" wheels as my normal place that I trust, can only do up to 18"
I went with the "don't watch" approach and survived to tell the tale. They were very good and very careful down at Demon Tweeks.