I see nothing wrong with using an impact to loosen... I even use one to put the lugs on (lightly), but finish up with a torque wrench.
That said, I stopped trusting shops after a near catastrophe. I had a shop put 4 new tires on right before a road trip. I was driving around to pick up some last minute things and heard a humming noise. At the time I didnt know much about cars so I took it back to the shop. Turns out NONE of the wheels had been torqued down. Again, I was ignorant so I just said, "Ok" and they torqued them down. Knowing what I know now, I would have been livid!
I just do my own stuff now, but I like doing it so its not "work" for me.