My association tends to be quicker on the whistle on a try than I would personally like, for the same "bad habits" reason others mention. We also make sure we give the "live ball" signal (like college officials do) for a field goal. I haven't blown one early on a FG yet, thank heavens...
In 5 man, if the snap is from inside the 15, or if we're in a "game critical" situation (late in the game with a tie or the lead on the line) we send the B and HL back, with the R covering the L's vacated side. If it's snapped outside the 15 and not a critical situation, B is back there all by his lonesome.
In 7 man, B and F are under the goal posts for all kick tries and all FG tries. The S serves as the second umpire on a try; on a FG, he has responsibility for the determining if the kick breaks the goal line, and lines up on the goal line at his hash mark.
In all cases, the "other official" is responsible for only his goalpost, while the B is responsible for the whistle, the crossbar, and their goalpost.