When I'm stripping down a car I investigate each area where I know I need to do welding or body work. If the area is in good condition (other than the bit that needs replacing) and it's not a bondoy mess I'll fi it before stripping paint unless it's something that I a) question or b) won't look right if done that way. I do the same with dents/dings/etc. That way, when you're sanding for curvature you're removing the old paint and not the metal.
Keep in mind that you will have to double check it all then the car is stripped, and obviously, you run the risk of having to redo.
Jody