I like the idea of putting the trim down first. That way, you don't have to worry about making a mess of the new carpet when you install the trim. Even if you pre-paint the trim, you'll still need to set and touch up the nail heads. Another big advantage to putting the trim down first is that you can do all your cutting in the room where you are installing, which to me was very convenient when I did it.
The installers recommended 1/4" of clearance. I went with 3/8" just to be on the safe side and it looks fine. I just ripped a few thin pieces of wood to the proper height and used them as spacers to speed the install.
Oh yeah, eventhough I installed the trim only a few days before they installed the carpet (so the paint was pretty fresh), they didn't damage it at all.
