Help with my first engine build (5.9 Magnum)

What will happen is you will sink the valves. This is bad….
Very bad for performance. You lose lift at the valve because it has to over come the amount you ground away. During the lift cycle, the valve spends more time in a low or dead flow area.

IF! After cleaning up the valves and seats the valve leaks, then replacing the valve becomes necessary. You may be able to get away with the pits on the valve in a cheap budget build, but do remind yourself new valves are not all that expensive in the big picture.