Everything is related. If you mill the heads, you decrease valve recession, if you recess the valves more (cut the valve face, or valve seat = valve job) you increase the "valve height", which then must be compensated for as 6.0's don't have adjustable rocker arms. So, you then must clip the tips of the valves down (where the bridges go) to achieve proper valve height. Without this, the pushrods would provide too much lifter preload. Also, but "sinking" the valves into the head, you're decreasing valve spring pressure, which can be an issue at high rpm, and high boost.
Do yourself a favor while it's apart, and have a valve job done (by a shop very familiar with these heads...if the one you're using isn't familiar that is). Trust me...I've seen it way too many times. Factory valve recession specs are .013-.027"...you can't be anywhere near .013"...I promise. You need to be at least .020...at the bear minimum, but most of these engines from the factory were set at .016-.020 so if you cut .008 off, you're at the limit. How bad would it suck to get it all back together and fire it up and then all the sudden loose compression on one or two cylinders because the valves hit the pistons. I've seen other machine shops that are good machine shops, if you have a small block chevy, or small block ford...just never messsed with this stuff before...it's finicky. Valve jobs on 32 valve motors aren't cheap, but neither is taking it all back appart and starting over. They can safely take .003-.004" off *most* heads, IMO, and that is (not to brag) but a very experianced decision. As long as your valve recession stays above .018-.020 or so, you'll probably be ok. If milling them .008 or more will put you closer to the .013 end, you need to "sink" the valves into the heads. You don't necessarily have to cut the valve seats, and the face of the actual valves, you could save money and relatively safely just do "half" a valve job... and just take .005 or so off the face (seat area of the actual valve, not to be confused with the seat in the head), and then readjust the valve height...which is another story. Valve height, measured from the spring seat pad (without the valve stem seal) is 2.220-2.225". I do so damned many of these I remember all these useless figures like this, but can't remember anniversary's and birthdays...lol. Anyway, if your heads have a valve height of 2.222, and you cut .005 off the face of the valve, and .005 off the valve seat in the head, you're effectively "sunk" the valve .010"...which needs to be clipped (ground) off the tip so as not to effect the rocker geometry. Now, that will change too as you mill tons off a head and/or block you have to compensate for that in valve height...it's all related, but that's way more in depth. Hope some of that made sense...
Check out the thread in my sig. Lots of pictures. You didn't get into the cracked issue, and the need for seats either...I bet they're cracked too. Can't accurately magnaflux (magnetic particle inspection that "finds" cracks") with the heads assembled. Anyway, Good luck, let me know if you have any questions. If you get me the measurements of your heads (valve recession before being milled, valve height, how much needs to be milled) I can help more. Sorry for rambling so long...I just really want people to understand this stuff...and it's one area I can actually really help people in. We build more than 200 of these a year (that's 400 heads, and 6400 valves, lol) we're busy.