You can usually get by with .030-.040 on the heads before you need to mill the intake. If it was me, I would not mill for compression.....although I have done it. Milling to gain compression means removing a LOT of material. Usually around .070" equals one compression point.
Normally, small blocks blueprint (actual measured compression ratio) very low. Oftentimes in the high 7s, like 7.8 or 7.9. Yes, they were rated at 8.2 and 8.5. Even the early 340 suffered this. They generally blueprinted down around 9.2 to 9.5, yet were rated at 10.2.
So, that means you'll have to mill about .140" off heads and or block on the average small block to get compression around 9.5. Then, you'll have to mill the intake to match. What you end up with is an engine that will only work with its own upper half. That is, unless you mill the block too, then you have a whole bastard engine.
What I recommend is blueprinting the deck height to spec. What is the spec? 9.6 ain't it? They are almost always high and crooked. The heads are much the same. The gasket surfaces on the heads is usually long. That means the combustion chambers are big.
By correcting all of this, you assure yourself that you have the deck of the block and heads where they are supposed to be. You also correct things like combustion chamber size and lifter preload, since you are straightening up the block deck surface. Some say it's splitting hairs, but I say it;s the difference between a good build and a not so good one.
Once you get those things (literally) squared away, you simply choose a piston to achieve the compression ratio you are looking for.
