Although I don't often do the finishing on interior trim work, I prefer to stain and apply one coat of finish before installation. Nail holes are filled with a color putty and the final top coats applied after it's up. We'll prime and paint the walls before the trim goes up, too. The job looks done, no fussy cutting in and it's much easier to do finishing on a bench laying flat than on the wall. Some painters would rather do it after it's up but paint and finish is a place where homeowners often invest their sweat equity and most are not pro painters.