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I almost never use a tripod, IS usually takes care of movement. I was tweaking the micro adjust of AF on a tripod recently and had to turn it off on my 300mm f/4 L and 100-400mmL.
With a super tele, even a heavy duty tripod does not stop motion, if I can't use a fast shutter speed, IS works fine. I had a older non IS 600mm L, and huge tripod, big Wimberly Head, perfectly balanced, weight on the underhook, arm over the top to damp out movement, and I still had to keep shutter speeds up around 1/100 and more, there always was some vibration.