Whether you reserve depend all on your travel dates. Travelling outside a holiday, in the middle of a week day on long distance trains? No need to reserve at all. Travelling around the Easter weekend, or in weekends during the summer holiday → best reserve on long distance/high speed services as a lot of people will be out travelling.
You won’t get stuck when you don’t reserve, no worries. That’s an irrational thought even. You’ll be travelling in countries with a very high level of public transport (in comparison to Australia). You really don’t have to be afraid that you’ll get stuck.
In the Netherlands, Germany, Austria, Switzerland you have mostly a minimum service of 1 train per hour, from 6 in the morning until 21h/22h (often running until later) in the evening, between the stations. On busier corridors much more trains, up to 1 train every 15 minutes between bigger cities and places. Trains have huge capacity, there’s always a place. If not, you just stand a bit on the train, wait until the next stop and take the seats of some people that get off at the next station or wait until the next train and have something to drink or to eat in or around the station while you wait.
The pass gives you lots of flexibility. You can always plan a bit of a detour on a slower, calmer, more scenic line, without the need reservations.
The only trains you might want to reserve a couple of months in advance are night trains in those countries. Otherwise reserving one or two weeks in advance (even one or two days in advance are okay for domestic services) are more than enough. And remember, there’s always another train ;)
