should i purchase train tickets online now for Rome to Chiusi (later taking a bus to Montepulciano)? I'm afraid they will be sold out if I don't... is this a good idea for Italy train tickets in general if we are trying to stick to a schedule?
It depends on where you are going and when. If you are going on a long journey (Paris to Rome, for example) I would buy tickets in advance. But to go throughout the small cities in Italy, there are a ton of inter-city train so you are probably fine purchasing as you go.
Most of the trains from Rome to Chiusi are regional trains. They don't sell out and there is really no advantage to buy in advance. You can only buy within 7 days of departure anyway. The Regionale Veloce trains are almost as fast as the Intercity trains (1 hr 50 min vs 1 hr 30 min).
This is our first time in Italy so I am not familiar with the train schedules... It sounds simple enough though! So just go to Termini station and we should be able to purchase tickets to Chiusi the day of? They won't be more expensive?
Check the roninrome.com site for step-bt-step instructions with photos. You can buy your ticket the day of your travel but you may run into some long lines, so you may want to buy it earlier. You can also buy your ticket from a kiosk and pay in euro (the machine gives change up to €20). The main thing to remember is to validate your regionale ticket in the grey/blue box at the train track before you board.
The regional train ticket prices don't change, it is 9.65 euro. More correctly, there are no advance purchase discounts. There may be some advance purchase tickets for 9.00 euro available for the Intercity trains, but they are not refundable and cannot be changed.