The guy that does my work has been doing it for 40+ years. The only way he will replace any sheetmetal is by having the car setting on its wheels. He says that's when it needs to look perfect, and that's the only way to get it that way. He has even told me that if shocks, suspension, etc need to be changed, to do it before he does the body work, because he wants it sitting the way it's going to ultimately sit. Don't know if there is anything to it, but he's been doing it for a long time, and it sounds logical to me.