I like to polish the entire valve head on the exhaust, it reduces the surface area and increases reflectivity and so keeps the heat retained in the valve down, at least thats my theory.

To do it I just chuck the valve in the drill press and run it on a slower speed, careful not to marr it up and dont grab it where the guide slides at all, then depending on surface texture of valve, start around 220 grit and work my way finer till around 2000 grit and I can see my face reflecting in it.