From experience, if you skim the block more than 1.5 or 2mm you need to load the head studs or bolts a few times before you do your finnal pass in the mill, reason being that it will make a slight buldge around all the head stud holes (besides oil hole) due to the slight change in loading area, also a good idea to add a tiny champher as the threads start to stop the top thread pulling up slightly.

I have seen people taking 4mm off between head and block, one must just remember to take into account what it does to the rocker angle, for most that run cut cams its actualy good because it makes it easier to get the perfect rocker ratio.