"Milling" the cylinder heads in order to increase the compression ration can be a good idea, so long as a couple of key considerations are kept in mind:
1) allow for proper valve clearance between their full extent, and the piston at its high mark. Custon pistons which have increased valve clearances are available to circumvent this issue.
2) more critically, however, is just how much pressure can be maintained before an explosion occurs, keeping in mind that structural integity is being decreased, while compression is being increased, beyond the cylinder head's original design safety factor.