I always use the highest that the monitor and videocard drivers will allow. The 30.82 Detonators will allow 120Hz max at 800x600, but the 40.xx will allow the monitor's actual highest refresh rate of 144Hz. It's not really a noticable difference, and I'm sticking with the 30.82's until the 40.xx's are perfected.
Anyway, what I run:
800x600 - 120Hz
1024x768 - 100Hz
1280x1024 - 85Hz. (Monitor supports 90Hz, but again, the 30.82's don't allow that speed for some reason.)
Monitor: Nokia Multigraph 447XPro, videocard: Geforce4 Ti4200 128MB