Thanks guys.

I'm actually not taking a local bus, I'm planning to take a medium-haul bus to Hakuba - there's a 3.30pm by Nagano Snow Shuttle which leaves from NRT T2, and a 4.20pm by JamJam which stops at HND T3 > NRT T3 (not T2 as I said earlier, I misremebered) > Hakuba. Their websites accept reservations, and it's >JPY10,000 one way so I assume most people would reserve?

I think I'll monitor spots on the 4.20pm bus - there's been 10 seats available for the past month, so if that continues I could potentially just make a decision only when I land.