What I usually do is rough cut everything as a bunch of angles on the seat (however many cutters I have that will fit in that particular hole) and do four angles on the valve. this establishes the basic seat area. Then, I use a flapper wheel to blend the cuts above and below the seat, and a hand-held abrasive wheel followed by a flapper on a die grinder and finally had polishing the valve with fine emery everywhere BUT the seat (takes a bit of practice to be safe at this!) giving an all-radiused set of faces either side of the seat. I no longer have to go back and do anything with the valve, but I usually put the pilots back in and touch up the valve seat (with carbides, not stones). The seat is identified when I lap, and I have never had to go back and move it either way to get what I want. Of course, I should add that I have been doing this for about 40 years, and it wasn't quite this easy when I first started. The secret is to slow down, be bloody careful, take your time and enjoy the process.
What most of this is all about is the airflow when the valves are close to the seat. If you look at a trace of valve lift vs. crank angle, you will notice that they move very slowly at each end of their travel. At the open side, not much other than the overall port makes any difference. However, near the seat, it is just that - the cleanliness of the flow near the seat, accross the edge of the valve and accross the face of the combustion chamber (the flow moves away from there at higher lift, but we are talking about a LONG time at very low lift). The results are quite easy to feel in either a gasser or diesel.
But, in referrence to this thread, you can't leave the cotact area to the intersection of two curves - it would then just be a very thin line and the valves will overheat (particularly exhaust). You CAN, however, narrow the seats a fair bit (intake in particular) and still get by with reasonable valve life and very good sealing.