Let me explain my position on this subject.
For years, every time I did a headgasket job, the heads went to the machine shop for resurfacing. Then a regular customer blew a headgasket on his Civic. I had the head resurfaced (it was indeed warped), and it went back together uneventfully. For the next couple of years, that Civic came back with repeated headgasket failures. Everytime I had the head resurfaced, and each time I searched further for a cause (all manner of cooling system checks: flow, rad, fans, ect...) with no real conclusion. Finally I sent the head to a different machine shop. I can't remenber the precise details, but they found the head to be not level. They asserted the other shop had resurfaced it improperly. The new shop resurfaced the head and the headgasket problem was cured.
I don't know precisely how machine shops resurface heads. I presume, like most things, there is more than one way to do it. All I know is that a machine shop with whom I had no other prior problems dicked-up a head resurfacing costing me hundreds of dollars and scores of hours.
I understand that this was a very isolated incident. However, since then, if a head is not warped, the mating surface is in good shape, and it is not going in for a valve job, I don't have it resurfaced.
My wife daily drives a '02 Disco and I daily drive a '98 Disco. They both have more than three years and I don't know how many miles since I did headgasket jobs on them without resurfacing the heads.