I’m below the spec and no problems

 

Sure, on many motors. No real danger of that on Rover 4.6's, though.

 

There is a small problem with machining alloy cylinder heads in that they are surface hardened to resist the heating and cooling cycles, the more machining they are subjected to the weaker the machined surface becomes. Having said that, IMO if needed I would machine mine for flatness but maybe only once, say to 0.010", TD5 heads are more susceptible to distortion. Fortunately I've never had to remove the head since new.