Being a trained mechanic and engine rebuilder I feel competent to add my input. I was taught that when it comes to lapping valves it should be done as a final step in the valve grinding to just obtain that final degree of perfection from a proper head rebuild. Using it to fix problems may invite some of the trouble already covered here. The seat width is considered very important so widening it by excessive lapping is not recomended. Most rebuilders make the mistake when checking the seat with bluing by rotating the valve , if there is a high spot on the valve it will appear as a complete circle using this method and lead one to believe the seal is good. Proper method is to just "pop" the valve straight in and out and check the pattern.
I was also taught that if the valves are not perfectly sealed the hot gasses will burn the seats and lead to failure.
There are enough variables and tenchniques that I can not cover here.