From: http://www.engintecs.com/html/how_its_done.html
When we are happy with the guides (your engine and you will be happy too), we move on to the seats . Seat width, height, contact point on the valve face and angle, all play a role in the cooling of the valve and seat, as well as breathing efficiency. Therefore, a Multi Angle seat work is performed, paying close attention to concentricity . Valves rely on the heat transfer through the seat for about 75% of their cooling A Concentricity Meter is the only way to verify the exact amount of runout between the seat and the guide and guess what : we are using one. Let's not forget that both seat and valve face need to be concentric, so valves are refaced using a Centerless Sunnen valve grinder, which is the best in the industry (holding .0004” concentricity or less) .
A final check of valves sealing is performed after their assembly by using a vacuum tester.