I agree with gy3. If you block sand the panels separate from the car the joints have a tendency to curve away from the panel joint. This is because the block is not guided across the joint and the edge of the panel gets sanded more than the rest of the flat or curved body lines.

You especially see this when the car is painted and reassembled where a sweeping bulge appears away form the joints. I have seen it many times where the panels meet the doors. I agree that it is easier to do the work with the car disassembled but the results should be the first concern.

I would insist on getting this done right from the start as after the car is painted and paid the bodyman gets amnesia about your car very quickly.

I kind of wonder if your affordable bodyman is worth it.