When is the best time to body work and finish the door jams? There is no convenient time to do that. What I’ve found to work well, and be the most efficient, is to do the jams last.
The outside of the body is done being block sanded, and essentially ready for paint and for all the panels to come off.
If the jams are done at this point, the body panels don’t have to go back on, saving a lot of adjustment time.
The big word of caution is to make sure the edges that make the gaps don’t get touched. Usually there’s some primer that is blown around the corner, which can be used as a guide. You can also use pencil on the edges, and make sure you don’t sand into the pencil marks. Also, tape just inside that corner whenever filler is spread. Do not lose where you gap edge is supposed to be!
With that being said, keep the doors and fenders on the entire time, take them off once, finish the jams, and paint!