In the RL550 I've had primers going sideways because of a dirty primer magazine (inner vertical tube) that creates drag on the primer vertical stack. But the horizontal feed system into the press completely isolates it. I think you could eventually prime the case with the primer upside down, or even sideways, but it never happened to me. In the event of a detonation, it would be well contained, and very far from the stack.
Yo could also manage to feed the primer magazine with primers facing the wrong side, but I've never done it, it is difficult due to the shape of the plastic part that catches the primers.