I have known a few people who have machined a very small amount off the
cylinder head surface.  If any significant amount is milled, the inside of
the combustion chamber must also be machined the same amount, else the
cylinders will not allow the mating surface to contact the head gasket as
firmly as is needed. 

What is normally done instead is to use a welder to add fresh aluminum to
the pitted mating surfaces, then machine back down to original
specifications.  This is what is done with most aftermarket rebuilt heads,
and often is done by local machine shops as well.  I had a set done by
Archway Auto Parts in St. Louis (a VW specialist) and was very satisfied
with the results.  

Bill
www.geocities.com/yosemite/gorge/1706