I'm another one who tends to leave IS On, especially with longer focal lengths.
Certainly it aids with handheld shooting, and also with a monopod. I have found that using it with a tripod can also be helpful, because as an earlier post noted, long focal lengths can show a lot of "jitter" through the viewfinder.
It should be noted, though, that especially with a tripod you want to keep the IS active (half-shutter pressed). If you lift off the shutter press, IS can de-activate over a few seconds, which can cause the lens/view to "droop". This can be quite distressing if you are shooting, say, moon shots (or other night views) where a close focus is critical at, say, 500-600 mms! In those scenarios I'm likely to turn the IS off on a tripod!