Well, It depends.. If you are traveling during the busy season, then you should make a reservation just to make sure you can get on the train (that it's not full). Some trains require a reservation no matter what. One thing to keep in mind, is when you make a reservation, you normally pay like 2-5 euro just to make it... So, if the reservation is not required, and you know there will be space on the train, then just hop on and go.. You need to get the Eurorail timetable book, it will list the times, and if a reservation is necessary.
I had the unlimited travel ticket, but some girls I was hanging out with had the 15 travel days one and they just had it stamped when they came by to check for tickets.
Also, if you can, get a night train.. You can just crash on the train and not waste a day in a train..