I leave it on most of the time too.. But through trial and error I found that you really do need to turn it off when on a tripod and if you are resting the camera on a good solid surface or you will get IS induced blurring of the image. I think the best example of IS induced blurring is trying to get a picture of the ISS overhead... You have a tiny bright dot surrounded by darkness and there is nothing for the IS to lock on to, so it jumps around and the IS makes the picture worse...or to summarize... "I have IS turned on, so why are the wings of the hummingbird blurred?"At the risk of being flamed, I feel that the many photographers around here that proclaim IS to be of no use on lenses wider than 85mm are being snobs. It's as if they are saying, "My technique is such that I would derive no benefit from it and if you feel the need for it, well you just suck." OK, that's a bit of an exaggeration, but you get my point.
I used to think the same way as you- the pros who suggest IS isn't important at wider FLs are snobs.
But let's dig deeper- there is SOME truth to it, as I have realized with time. Not all true, mind, because I still think IS is important.
However, I think IS gives a false sense of confidence to inexperienced photographers. They feel they can shoot a photo at 1/17 just because they are shooting with a 35mm lens with IS. But they don't understand the limitation of shutter speed vs subject movement.
Pros point at the fact that you realistically cannot shoot lower than 1/n (put your favorite number here) unless you want motion blur or you are shooting still life.
Now, for longer focal lengths, n is a larger number:
Consequently 1/n is higher, and 1/n divided by factor of image stabilization still remains high. So motion blur is avoided.
Now, less knowledgeable people have taken this maxim, misunderstood it, and propagated it at face value- that IS is unimportant. I think it is just a misrepresentation and generalization of otherwise sound logic.
To me, IS is a tool. Sometimes it is needed, sometimes it is not. The trick is knowing where and when.
I agree to the extent that IS is frequently not needed at these FLs, and that it certainly is not a cure-all for poor technique. However, I have made quite a few shots where IS has undoubtedly helped, and have never had a shot ruined by it. I leave it on always, needed or not.
Point taken. Since I was relating IS to 85mm and wider, I was referring to hand held shooting. Though the majority of my subjects are closer than 250 miles, I shall keep your good tip in mind