"400 is in the tempering range"
This is true, and the reason why I advocate using that temperature. It will be the minimum temp that chisels will be tempered at, and that would be if they are a medium carbon steel. Higher carbon steels need to be tempered at an elevated temp in my experience. Heating to that temperature will not open any can of worms with the steel, but you might not want the handle on it when you do it.
" if you are going to go that far, you could just reforge the end and retemper"
Partly true, and reforging would indeed be way too much work. Again, the temperatures that I mentioned are not sufficient to remove the temper from the chisel,(BTW, what Mr. Davidson is doing up there is forging, just not at a high temp) Typical ranges for hot forging carbon and low alloy steels is in the range of 1600 to 2300 degreesF, a far cry from what I had mentioned.
Believe it or not, a standard practice in industry to relieve grinding stresses, ect, is to bring the part up to the final tempering temp and hold for an hour or three.