I’ll be wanting to train from Munich to Salzburg (and back) in mid June. Is it necessary to purchase tickets in advance? And if so, how far in advance? I’d like to leave it to the day before if possible, in case we want to spend part of that day in Munich rather than departing first thing in the morning.
It is not necessary to buy train tickets in advance. If you were to go to the station right now, you would be able to buy a ticket and be on the next train. Tickets are for a route, and never sell out. Of course it is possible that more people board a train then there are seats, and then some people stand, just like on a subway.
Buying your ticket in advance however does give discounts. And reserving a seat in addition to your ticket (only possible on some trains) guarantees you a seat.
You have three options: (1) local transport (BRB): hourly, usable with Bayernticket after 9am (or anytime on weekends; €32/1, 42/2); no advance booking, no seat reservation; (2) ÖBB RailJet: hourly; walkup fare approx. €58 pp, advance booking approx. €30 pp, seat reservation for a fee; compare fares on bahn.de and Oebb.at. (3) WestBahn: five trains per day; walkup fare approx. €50 pp including seat reservation; if booked in advance from approx. €20 pp.
The easiest way to buy option (1) is at a ticket machine or via the DB Navigator, (2) via the DB Navigator (no saver fares at ticket machines!) or oebb.at, (3) at westbahn.at.