Originally posted by Lowell Goudge for IS at high shutter speeds I am almost of the impression that once you are over 1/(focal length *1.5) IS is not needed.
Also note, IS is NOT used during flash photography, or tripod use or when you are panning the camera to track moving subjects. In each of these cases IS will interfere with the image negatively, and for flash, the camera automatically shuts it down.
On my K10D, SR definitely remains active with the built-in flash turned on. I just took a shot and checked the EXIF. In addition, the little hand appeared in the viewfinder, indicating that SR had stabilized.
I'm not sure how SR would interfere with the image. If you're shooting a normal flash, at or near the maximum sync speed of 1/180, it may not be necessary, but I don't see how it could detract from image quality.
If you're dragging the shutter to bring out shadows via ambient light, SR would definitely help.