I had a similair question that I posted ealier.
If you want to be flexible with when and where you go, then the advice I was given was to reserve them as you travel. Such as booking your first couple of nights before you leave for your trip and then visit interenet cafes along the way and book a day or two in advance as you travel for the upcoming nights.
If you are traveling by train then it is best to catch the night train so you can sleep on the train and not have to pay for a hostel that night. Plus, you dont have to spend time that you could be site seeing on a train.
But if you have a pretty rigid outline that you want to follow everyone said to book all your hostels ahead of time. Make sure you check to see if there is a refund or cancelation fee just in case.
Hopefully that can answer at little bit of your question.
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