Once lost a job to Bob Cupp trying to honestly answer this question......
The pro types do like 2-3 feet, believing anything further doesn't "come into play." Obviously, with the newfound PGA tour emphasis on tucking pins close, the closer the better to separate good shots from bad (at that level) At lower levels of play, we don't want to get them too far from the green in most cases because it just makes for a tough bunker shot, but I can see leaving a few more feet of room for average play. Think about it - if you have a circular 6K SF green (about 90 diameter) adding 3 more feet all the way around enlarges the target to 6900 (15%) and 8 feet all the way around enlarges it to 8425 SF (40% increase from the green, and 20% increase from a 3' collar)
IMHO, that extra space keeps a lot of balls from finding bunkers, slowing play, etc. It also equates to that triplex turning distance at 8 feet, so I generally leave my bunkers about that far from the green.
I find that I have to draw the 6 feet away (see the Ross thread on drawing plans to get them built as you want, not as depicted) because both green and bunker are cavities, and they tend to start cutting from wherever the construction stake is, and losing a foot or so from the intended dimensions.
I had a super recently tell me that ten feet from the edge is required for triplex turning. Not ready to do that, although it has happened as some of my draftsmen like to graphically leave a bigger space just for plan looks. That drives me crazy!