Cylinder head milling

Just use a thicker head gasket.

If it fits, it sits.

But there may be some logic to it. It's been awhile since I've read anything about cylinder head machining. I need to find my books of automotive knowledge.
Depends on the why the block or head(s) are being milled. Could be warping and mill to get it flat again. If milling to zero deck or increase compression there is a good possibility more than a few thousandths is removed. An engine that has bbeen remanufactured a couple of times may have enough cut requiring milling the intake face of the intake and a different amount taken off the china wall portion of the manifold.