Agreed, Grinding on the mill or lathe of hard surfaces is a great idea.
Trying to hold surface grinder tollerances, Is not. But the two things are totaly seperate.
You might of just hardened something, And only need to grind it within 0.001" after hardening distorted it. Or the grinding might just be to remove scratchs and other defects from a surface that is too hard to mill, but again does not need absolute precision.