Yep, loaded question.
Me, not being a pro painter, will 'jamb' it with color then do the CC over everything. But with that said, it will depend on the painter and his personal technique. What you are really looking for is color over the entire car and the slickly applied CC over the external panels while the jambs, firewalls, trunk inner panels usually aren't quite as important and get cleared while doing the rest of the car. Then there is, as you said, possibly masking off and that dreaded line where you taped and now you have to cut and buff out. No way would I try to sand a car, leaving more dust or sludge after a jamb BC/CC, try to clean then do the rest of the car external panels.

As you see, no easy answer - it's what works best for you - and hope this wasn't too confused