Interesting topic. For me and my 13.6 USGA index, I've found that length off the tee is not the problem so much as the longer shots into the greens and the tough recoveries after missing said greens. At my club I usually always play the blues (6583 yards). There are really only three holes where I'll play the blues differently than the whites (6285 yards).
Yardage isn't everything of course, but in general terms I've found that I can not make a fool of myself up to around 6800 yards. Of course a lot of it has to do with were the yardage comes from. Long par 4s (over, say, 430 yards) or long par 3s (greater than 180) cause much greater problems than 570 yard par 5s do.
On Monday I played 36 at Southern Dunes outside Orlando. The first 18 I played from the blues (6800) and shot 99! The second 18 I played from the whites (6300) and shot 91. And the 91 included a couple of bad scores due to "going for" shots that I usually wouldn't go for.
Interestingly enough I parred the #1 handicap hole from both the blues and the whites. Driver, 4 iron from the blues. Driver, 6 iron from the whites.
I don't have any firm rules but as guidelines I try to stay under 6700/6800 yards, 130 slope and 72 rating. Once in a while I'll step back to the 7000 yard tees and have a go at it. I usually regret it.
I apply the same principle whether I'm playing my club, a local course, or a "special" course - Cuscowilla, Tidewater, Southern Dunes...
TimT