I've never done it on a lathe, but I think it would be much better accomplished on a valve refacing machine. I think the material valves are made of would be more easily ground, than cut with a tool.
The only issue with this is the heat from the stone and stone wear.
If you are using a water based coolant (I'd much rather use a WB coolant than oil any time I can) you only have to deal with stone wear and on 16 valves with truing you'll pretty much use up most of a grinding stone right there.
