From a collision repair perspective:
1. Maintain fit of doors (and all associated panels) during repair process, yes, 100% necessary.
2. Making numerous screw holes in outer skins of cosmetic panels, no, avoid if at all possible.
SOP in the shops I have worked at would be to use the hinges and perhaps even the latch/striker to locate the door*. If the lower hinge mounting of the lower pillar is to be cut out and replaced, do careful measuring with a tape measure (or better, tram gauge, which I think is a good investment for a builder) then hold the lower hinge section in place with any combo of clamps, Clecos, screws, and tack welds, then try the door on again. At that point more welds can be added to the hinge pillar once the the fit is satisfactory. Watch the fit as you add welds. Typically the door opening will tighten up just a touch during welding, so typically try to be on the big side of just right with your gaps.
Of course, it's pretty normal that the tacks will have to be cut and the lower pillar moved at least once before final welding based on the fit. But you don't want the structure under any kind of tension during fit and welding if at all possible, which is another reason why having the door semi-rigidly connected to the door opening with straps might not be the best plan. You could encounter a situation where the door jumps to a different alignment when the screws are removed, and belatedly realize that the pillar isn't quite where you wanted it.
I apologize for the direct criticism Dean, I mean it only in a constructive way, so to speak. And I welcome criticisms of my criticism from all.
*Striker should be removed for most of the process, it gets put in mostly to assist in the mounting of the door.
