I've had good results just lapping in the new valves with out remachining the valve seats. test fit your valves in the guides that are in the head now. They should slide easily with a spot of oil.
You could try all new valves, reaming the tight guides, and lapping in the valves. Probably not the most correct way to do it, but it has served me well. If you have a couple seats that don't look like the contact area is wide enough, just remachine those.