I think a good question in general is whether there is any change in IQ when the IS elements are shifted to compensate for motion.
It's a good question but different from the original one I asked. In my case the primary subject looks perfectly sharp with IS on. The background under some circumstances would look a little nervous, a little busy.
If the IS helps make the subject sharp but I have to sacrifice the smoothness of the background, I may want to turn IS off and use it very sparingly. If IS doesn't really do anything to the background I could leave it on at all times. Ever since I have owned my tele I have never turned IS off.