For static subjects, including birds/wildlife, I prefer to use a tripod. This is especially important if the subject is small in the frame and will require significant cropping. It also helps if the subject is very close and the DOF is minute. The tripod will not only reduce vibration (if used with mirror lockup and a remote release or self timer) but will also ensure you do not change the subject distance between focusing and taking the shot. In my experience, hand holding can result in focus being shifted between focusing and firing, and even a couple of mm can be the difference between success and failure when shooting at close range.
Here is an example where I was shooting at close range with the aid of the tripod. The day before I had tried similar shots without a tripod, and even though seated and using IS the results could not compare with the tripod shots. The hand held shots all ended up in the bin. If you look at the crop from this shot you may notice that the DOF is tiny. The feathers above the eye are sharp, but those on the cheek are not. You can throw out your DOF tables when it comes to truly checking accurate focus. Close up, every mm matters.
Full image, unedited :
100% crop :
For active subjects I prefer to shoot hand held. Whether I use IS or not depends on what kind of motion my subject is in and what sort of shutter speeds I'm using.
Given that the aim of IS is to stabilise the lens, if you have an erratic subject or one requiring a diagonal pan I think it is a problem for IS to help you. On my 100-400 I have noticed IS fighting with me as I attempt to follow a tricky subject. For such subjects I will disable IS. If you can perform a smooth horizontal or vertical pan then IS (panning mode) may help. If you are trying to keep the camera absolutely still then full IS can be advantageous. However, at shutter speeds above 1/800 I tend to disable it, and for BIF I would usually want to be shooting at upwards of 1/800. Not always, but usually.
So, use IS when you need it, but turn it off when you don't. It's a bit like filters. Use them when you need to. Take them off when you don't.
p.s. for Nikon shooters you may find this article interesting -
http://www.bythom.com/nikon-vr.htm. His thinking may have relevance for other brands too.