Mill head or block or both?

If you are only taking the head off and not doing any block - bore work just mill the head.
If you are doing a complete rebuild there are some advantages to taking a cut on both.
Do the block machine first, you will want to have the pistons you plan to use in hand so the machine shop can get the bore size right.
Get the bore and any crank work done then get the block back from the machine shop.
Install the crank with the bearings. If you are using stock style press in pins, have the MS turn one piston pin down so it is a press fit. Then install a piston, no rings necessary in bore 1 and get the piston recession number. Use the same piston and rod in bore 6 and get the piston recession number. Be careful here, some piston top have a slight crown to them. Put a mark on the piston top with a magic marker and measure from the same spot on the piston each time. This will tell you if a front to rear angled clean up cut on the block is necessary to straighten up the block. I normally cut .010 on the block to get a new flat gasket surface
Using what was cut from the block and your new piston you will have the piston recession numbers to calculate what needs to be cut from the head to hit your target compression ratio.
Last but not least, machine shop hot tanks do a good job cleaning the exterior of blocks - heads- cranks and depositing debris in the oil lines. When you get those major components back from the MS, prior to assembly use plenty of hot water, Dawn dish soap and the right size and length of bottle brushes to scrub all oil lines and bolt holes. Use compressed air to dry throughout and wrap each in a large plastic bag till it is assembled. Eastwood has a water soluble rust inhibitor that is good as a final rinse. The parts still need to be 100% dry.