It depends a little on what time of the year you are travelling and whether you want to stay at a specific hostel or not. We booked all of ours in advance because we knew where we were staying. We did see a few people show up at the hostel and be turned away because it was full. Really up to you and what you can live with.

Well, personally, I like to book in advance. Particularly if it is summer, as that is high season, but even if not I still like to know where I’ll be staying. Also, I want to stay in particular places (that have high ratings on hostelworld/hostelbookers, have breakfast incl, have laundry and cheap internet, etc), I’m kinda picky… so I want to make sure and get the hostels I have picked out. Also, for my trip this fall, I am going only for two weeks and have the itinerary pretty much planned out already so as not to waste time, so I will be booking pretty far in advance for both hostels and buses/trains.