i am wondering if there is ever a good time to use the OIS in a moviemaking context or if its like AGC
obviously i dont want it with tripod shots but if handheld / shoulder mount and so on - am i better off turning it off and then smoothcamming in post if necessary?
It will only get rid of little shakes, but Canon's OIS is really well done, imo. It's a really noticable improvement if you are on the long end of the zoom.
As for smoothcam, it is a great effect, but use it only if you have to. In order for it to work, you have to digitally zoom in on your video. This will, of course, even in the best case scenario affect the quality.
Optical image stabilization doesn't degrade the image. It is actually moving physical elements to reduce vibration. Stay away from electronic image stabilization, though, as it does and is in some consumer cams.
By turning the OIS off, you can avoid issues like the camera compensating for movements you actually intented to make, etc. You will probably also improve your battery life, but I don't know by how much.
So, anyway, if you don't need it, go ahead and leave it off, but don't be afraid to use it as it works great.