I used to turn it off, but I forgot to do so on my laptop, and I had a bad installer bork the system. Rolled back to the restore point that was auto-generated earlier that day, and all was well. I didn't reinstall that program. I leave it on now, and manually clear out old restore points and create new ones. I also usually create an restore point when I install something that has potential to change the system.
Those of you who use imaging tools, do you make a completely new image (maybe) spanning several DVDs whenever you install something that can change the system, or do you just reload the image and spend hours re-installing everything not in the image?