I can see that grinding the end of a valve stem square without the vavle grinding rig would be a tricky buissness.
On the subject of case hardening. I throught that I was not good in high temperature applications and that things like cams lost the case hardening when they get too hot. That and the fact that all the best engine machien shops I know grind valves makes me think they are not case hardened.
I really cant find anything about how valves are made on the interweb. I love the mad videos of stuff being cast and machiened.