Involvement with 'the body work' will be subjective, of course. The painter may or may not need to follow-up your body work with some adjusting or tweaking of their own. They also may (or may not) need to remove components to ensure everything gets painted.
For that reason, I took my car to the painter unassembled. They needed to repair the doors and a few thin spots on the hood, along with the seams where the quarter panel skins joined the original sheet metal (there's some filler of course, but they worked the metal and got it a lot closer than I could).
Sadly, the body guy didn't get everything 100% lined up as well as it was beforehand, and as a result there are a few minor things I've noticed that most people generally miss, and I have just chalked up to 'added character.' I'm just really happy that the things I did to it turned out as well as they did... although I'm sure a professional with an interest in our cars would probably spot the issues and wonder, "what happened there?"
Based on my experience, I would advise anybody to pick an established body shop (one that's not going to be at risk to close up shop while your car is there because the body guy smokes dope), and to 'be involved' as much as possible while the car is in their care. Go to the shop, inspect their work as it progresses, and don't be afraid to point out potential issues along the way... it's easier to correct them before the paint goes down, after all.
Get an estimate... insist on it, with the understanding that the price may have to go up if more work and/or materials are required - and make sure the shop "runs it by you first" before authorizing more work that will require more money.
Have an inventory of the items you take to them - otherwise, you may be dazzled by the awesome paint job when you pick it up, and leave while forgetting to take something important (but not outwardly visible) with you.
Overall, I'm happy with mine, and I don't hate my painter or anything - but I'm sure at some point down the road it'll need another paint job, and I already have a list of things that will need attention for the next go-around.
I know it's probably not exactly the information you're looking for, but I think it'll be helpful for when it's time to pick your painter.