Metal gets work hardend from repeated hammering. I suspect that if the seats don't get hot enough to anneal then they are hard enough to be safe, if they had enough miles with lead on them. The seats on my engines are fine and I never run them with lead. I figure the rings are going bad before the valves anyway, the way I drive. I watch for seat recession by checking the valve clearance. If they get tighter over time the seats are burning but if they get looser the valve train is wearing.
