I did all of my repairs off the frame on a dolly and it seems that I did not have any alignment or fitment issues after everything was back together. I think a lot of people may point you to do it on the frame, which is probably better but like you said access might become an issue. If you do decide to do it off the frame Install some bracing in the door frames and an X brace between the corners to keep it square. Big plus to having it on a dolly is you can move it around, push it off to the side of shop when not working, roll it outside if it's nice out, etc. I guess the best of both would be to build a dolly with mounting points on it in their correct locations.
