Often he will spread us out in groups down the mat to practice a technique. He might walk down the mat, away from the shomen, to watch the groups in back. The pairs in front would inevetiablely be between him and the shomen. Is that OK since they weren't the ones to move there?
In my (limited) experience I would say yeah, that's all right. That is sensei making that choice. He can't do his duty of teaching with out that after all.
I liken it to the whole "no backs to O Sensei" rule. During insturctions (no matter how informal) NO ONE sits with the shomen/place of honor behind them... but occasionally, thru the process of practice somebady may have the shomen behind them. (in fact there was a thread a loooong time ago that had people describing how they accidentally fell into/onto or up against the shomen
) [however, I realize that at other dojos this "no backs..." rule may be incredibly serious and they have a way of practice that assures this is always followed....]