You'd be surprised! I have had plenty of paintless dent repair on 4 different vehicles and not once did the guy remove any panels. He had some very specialized tools that allowed him to get into seemingly immpossible spots. On one of the cars, he did have to drill a small hole in the inside of the door jam to get to a dent on the rear quarter that was otherwise inaccessible.
Make sure that you find a very experienced guy, though, as it truly is an art form.
While getting a large dent taken out of my wife's car, the PDR guy pointed out some previous work on the car that had been done by a PDR newbie. It was quite obvious with lot's of little bumps where the guy pushed the metal out too much and with the wrong tool.