I built my Roubo about 12 years ago. I've flattened the top twice, I didn't pay any attention to grain direction when I glued up, and unless you are using really nice stock and throwing stuff away I don't see how you are going to end up without some reversing grain. And it doesn't matter, because you do the bulk of the flattening by traversing across the top anyway, and then if you want to smooth it out you have plenty of room to maneuver with different grain direction. And a little tearout will have zero impact on the bench in use.

I also suggest doing the 1 board at a time glue up, and when I do my next bench I'll probably add some small festool dominoes to help with alignment. You could easily do that with hand tools, adding a little mortise and small key on each end. It will definitely help my sanity in the glue up, because you are going to want full glue coverage and once you start clamping its going to be sliding around. Anyway, that was my experience on an 8 foot long bench.