After the seats are ground and the valves refaced or replaced -- I always fit new exhaust valves -- I use bluing (AKA: Prussian Blue) to check seat contact. Three dabs around the seat, spin the valve and all is revealed.
I'm old school, so once I'm happy with that, I very lightly do a valve lap at the end. Most machine shops cringe when I say this and I think they are probably right. I started with high-speed, valve seat grinders, now they use special cutters, much more precise.