tlorber: The instances where I've seen a top end rebuild (used to be called a "valve job") cause problems is when some sort of performance enhancements are used (heavier valve springs, longer ratio rockers, increasing compression ratio, etc.) without an accompanying refresh of a tired bottom end.
In general, it used to be pretty common to grind valves, knurl or replace valve guides and replace the head gasket without opening up the bottom of the motor. In my opinion, it's only when additional stress (higher compression, stiffer valve train components) is added to the motor that problems might occur.
hth,
