Is there a huge cylinder head flow difference between 3 angle vs. 5 angle valve seats....? Not so much.
I've spent countless hours on SuperFlows, from the old tube style pressure readers, to the new "touch one button" setups playing with all kinds of cylinder head setups. I've only seen some minor gains here and there, typically in the low lift numbers with different seat angles, including radius'd exaust valve heads flowing them at 25 and 28 inches of water. I know some guys that would pay big bucks if you could get them an additional 7cfm's out of their heads.
I reread that and it sounded like I don't condone 5 angle seats. If you have the cutters, by all means multi-angle cut the seats, backcut your intake valves and radius your exaust valve, but don't expect huge gains. Put your trust into someone who will make sure you have a good set of guides and knows how your seat width will work with your particular combination. And lastly, lap the damn things in. They only have to touch them for a second to see where the valve hits the seat and it saves the hassle of getting an uneven seat or valve face because the cutter chattered the seat. Even with a Sunnen automated cutter you can still get crappy seats. The machine work is only as good as the guy sending it out the door.