Consider that a low temper may be 175-200C for extended periods (well over an hour). A brief exposure to boiling water is no big deal. Even when you
Blade Guards : http://www.amazon.com/Kitchen-Supply...de+guard+12%22
+1 to everything Rick and China said. I've used and sharpened, but not owned, a Kohetsu AS and do currently own an Akifusa (can't comment on the Tojiro).
btw, what is the maximum temperature one can heat the knife to without ruining the HT? Is 100C (boiling water) hot enough?