This is one of those topics that has brought about much debate for some reason. Valve Refacing Machines use Chucks to hold the valve by the valve stem and they have what is called "Minimum Valve Stem Length" that they are able to hold on to. But how exactly do you determine if that "Minimum" is going to be good enough for your needs if you don't know how to correctly measure the valve stem in the first place?
Not to worry. This image below says it all for you.
The Minimum Valve Stem Length is the distance from below the "keepers" groove to before the taper for the Valve Head begins.
This is the area where the valve chuck can grab onto the valve stem to hold it securely during a grinding operation. And there are several different schemes for holding a used valve in such a way that the "centerline" is correctly achieved.