IS is designed to help with lower shutter speed images, ie. low light levels. IS will not help you at all in bright sunlight as the shutter is so fast you will not need it. the reason the safari photo posted below is so sharp is not because of the IS, it's because the shutter speed is fast (1/200 sec) due to the bright light.
IS is not designed to counteract shaky ride vehicles or anything else other than lower shutter speeds. it works (in my canon slr lenses) but having one of the lens elements inside the lens "float" and uses ultrasonic sound waves to move this "floating" element into the correct position so the focal plane always remains the same. i have been able to hand hold the camera down to 1/10 second and get a sharp image, which is virtually impossible without the IS.
older IS technology MUST be turned off when using a tripod as it causes the lens to vibrate which the IS system is working. for this reason, the tripod will actually counteract the IS, causing the image to not be as sharp as it would be otherwise. with a tripod, there is absolutely no need whatsoever to use IS, as that is what the tripod is for to begin with.
so, to answer the original question, just having big unsteady hands is not a reason to require IS. if the shutter speed is high enough, you won't need it.