I won't do it. Either I'll straighten during the fall in temp during martensite formation, or it gets the 3 point clamp method during tempering. If I can't get it out either way, I start over. And the only time I've had to start over was just recently with AEBL chef's knives. I couldn't get them straight enough to my satisfactory.
My only experience with torched spines for straightening was with Peter's HT. I had a few AEBL kitchen knives that came back with the spine/torch method. These were at final dimensions and polish. The affected area was clearly visible by the oxides left over, which were easily removed with a final 800 grit polish. And the affected area was about the size of a pinky nail...a small semi-circle that extended down from the spine about 3/8" or so. Honestly, on a chef's knife, I don't think anyone would ever notice any change of any kind. A stainless steel knife that was going to be used for more rigorous duty than a chef's knife, like a chopper for example, I would not want to use the torch on the spine at all.