Is there an advantage to using M8 screws for the 608 bearings vs 5/16"? I would think you want to get rid as much play as possible (leaning towards M8).
Nope.
I live in metric hemisphere and built the eu version of MPCNC. 5/16*0,254=7,94, that’s slightly less than 8mm. However in my experience the parts around x and y pipes are quite a bit smaller than they should be on paper, so the rollers will need to be forced on the pipes and they will elastically bend when in place, actually compensating for much more play than this difference. Moreover, the slight emboss around M8 screws holes will help centering the bearings (and keeping their outer ring from interfering with boxing), so I guess things would work even with much smaller and lighter screws.
Happy making
Marco