I mostly leave the IS on, or turn it on again immediately after tripod use, because I've found that I'm so used to its beneficial effects that I end up wildly overestimating my ability to shoot handheld if it's off. I produced a whole bunch of blurry shots one fine morning because I thought I could easily handhold 1/30 sec or thereabouts - which I can, with the cheapie nifty fifty. With the 70-300L, not so much...