From: http://www.automotiverebuilder.com/ar/ar119854.htm

“Valve jobs are like a number of other automotive machining operations � there�s a right way and a wrong way to do them, but what�s right for one application may be wrong for another. Although a standard “cut and buff” approach is entirely satisfactory for most car and light truck engines, performance enthusiasts invariably look for something better.

That �something� is usually a multi-angle valve job. And for good reason; on a typical V8 engine, it�s worth about 10 horsepower. A good multi-angle valve job (three angles are most commonly used) includes a 70-degree bottom cut, which is the same angle found in many stock heads. But as opposed to the 70-degree cut meeting the 45-degree cut of the valve seat, a three-angle valve job incorporates a 60-degree cut between the two cuts to smooth the transition from the valve seat as well as to narrow the valve seat.

The seat itself is cut with a 45-degree cutter, while a 30-degree cutter machines the transition from the top of the valve seat to the combustion chamber surface. The accuracy of the valve job exerts a significant influence over performance because it affects airflow, as well as the quality of the seal between the valve face and seat.”

More in-depth info can be found at the link above.