Japan is the country I nonrev to most. I don't think I've ever been asked to show a return ticket there. In places where they do ask, the printed return "reservation" is good enough. Funny you should ask this, in that when I travel by myself, I often only make one-way reservations and don't book the return until some later point. I get detained in the UK by immigration every time I do this, so having the paper in hand saves drama.