I have never been asked by an immigration agent to show proof of onward journey for any country. However, JAL is the only airline that regularly challenges me at checkin to see such proof if I have a one-way or other unusual ticket where it's no obvious I'm going straight back to my country of citizenship after visiting Japan. I would recommend having an onward ticket booked before visiting any country, but especially Japan. If you're unsure of date, just book something refundable/flexible (e.g. a mileage ticket) so you can change it once you know.