Git still doesn't record the tip in the last rebase (or merge), but instead tries to calculate which commits are different all the way up to the common ancestor. If the roots aren't the same I don't think it can rebase without --onto. And even if they share a commit somewhere far back, my experience is that it can easily snag on some old commit along the way, and one will end up with a complete mess. So I'd use --onto in any case.
Not that it's particularly difficult or anything, but it requires a bit of coordination so that people don't push any old branch tips and thereby pushes the whole old repository back in.
