I had a machinist tell me that lapping in shouldn't be done because even though the valves fit when they are cold from the procedure, when the engine reaches normal operating temperature, the valves don't seat properly any more, or at least not as well as they should from a proper grinding procedure.
The other problem is that the small bits of carbide in the lapping grease can get embedded in the seat and valve face and no amount of cleaning can get rid of this.
These bits of carbide cause hot spots on the exhaust valve eventually causing pitting and premature failure.
With that said, I've been lapping valves in too and the given engine runs fine, it's just that this something that has always stuck in my head.
Found this, I guess it depends how fussy you are and how deep your pockets are... (the long post and the photo after it)