The valve faces were hardened down several thousands deep, called stellite. You can safely grind off a couple thousands (.001") and still have the stellite surface, much more and it's the softer steel that'll hit the seats. After several thousand miles of riding, those valve faces will be concave, they'll still work, but won't be as tolerant of overheating.
We always did, as a minimum, a light lap with the finest compound. The grey frosting on the valve and seat was used as a visual confirmation of valve/seat matching. If you can't see this, just use the sharpie or marks-a-lot on clean/dry valve and seat, lap without compound, inspect the marks. Final leak test, done a variety of ways, involves assembling valves/springs/keepers, brace head up, pour solvent into ports, leave it for 15-30 minutes, look for weeping/moisture/runs around valve faces.
