If you're exposing for more than a second or so, it's better to turn off IS altogether. If you activate IS for several seconds while watching through the viewfinder, you see an initial "jump" when it first comes on, and then the image will slowly "wander" around. For long expusures this can result in more blur than if IS is off. But for anything faster than 1/5 or so, IS on will either reduce camera shake blur or at least not worsen it.
Jonathan, is this a fact or an educated deduction from your experience, if fact is it documented some where? I ask because I have just started using a 600 with IS and have got some bad results of a few photos that I thought I was using all the correct methods, But I have been exposing for longer than 2 seconds which would make some sence. I will certanly be trying without IS to see how I fair. Following a bird using an extended run of say 12 exposures over 3 or 4 seconds does not seem to cause a problem with the IS running for this time though.