In a perfect world, you would thin every time you sharpened your knife. Realistically, this doesn't happen. Most of the knives we deal with here are so thin behind the edge, that the time to do so, (leaving the knife face in the condition we got it) would outweigh any of the benefit you got from doing it. The ITK is a perfect example of this. And for a reference, here's a shot of the choil:
In all reality, you can go quite a while before you would notice any performance degradation without thinning the knife. Just because it is so frickin' thin behind the edge. It's actually more efficient to just wait for a while before attempting to thin the knife with the amount of time you would spend on the stones to get the finish back to where it was. It does become somewhat of a guessing game once you wait that long, to thin it to the right angle. But I'll try to draw something up tomorrow to better illustrate that point. For now, I would suggest just touching up the edge when you need to, as this will help you get a feel for keeping a consistent angle. Once you start to thin, the feedback is a bit harder to gauge.
The reason you don't see any scratches on the knives any of the pro's sharpen, is that they work through a full progression of stones, essentially re-polishing the knife as they've taken metal away at lower grits.