I think I’m following you. Yes, since the cutter spins in a circular motion, it will have slight cut marks from the front & back arc of the circle.
So, I bought a used '82 head from a 60 which appears to been decked, post-manufacturer. Using a Starrett combination square and a feeler guage, I can measure an average of .001" dip across, from plug side to manifold side. The tooling marks also suggest that the milling head was not 'trammed,' because there is no cross-hatching on the long edges of the head. If the corresponding 2F block was treated in a similar manner, that would leave a compounding dip of similar value, adding up to about .002." The spec for flatness is about .005," if I recall correctly. So, being about 2/5 of the way there, it leaves significantly less room for thermal warping, effects of mill tool or flycutter 'rake,' or, slop on axis of the mill. I've never worked in an automotive machine shop, but I spent several years working on Bridgeport mills, which included 'tramming,' or setting the spindle perpendicular to the x and y axis of movement; it was hard, partly because these machines were worn out.
Anyway, just because the block or the head was decked, I would not assume anything. And given the opportunity, I would try my hardest to machine, and measure flatness, knowing that this will leave the greatest amount of load shared with the head gasket in the particularly narrow region between the cylinders.
Cheers,
Paul aka 'Dizzy'