On one entry out of 8 to NRT during last 12 months I've been asked to show my return ticket. And that was when I entered Japan second time that week. And of course that was the only time I didn't have my ticket with me. I was heading for a holiday and all I had was a printed itinerary from expedia web page. Not sure if it had ticket number in it, but they didn't ask anything else. Of course that's no guarantee for any other occasion.