Do I need to pre-book hostels?

It depends on how much you want to stay at a particular hostel. During the busy summer months of December to February hostels can get fully booked although dorm beds are easier to come by than private rooms.

In some places, especially larger towns and cities there are several hostels all within a short distance of each other so if one is full you can try another. In more out of the way places however there may only be one hostel and if you arrive to find that full you may have no way of getting to somewhere else.

Most of the hostels can be booked ahead either online or by telephone and many people prefer to do this to ensure they will have a bed for the night. If your itinerary is fairly loose then you may only wish to book one or two nights ahead while other people plan their whole itinerary weeks in advance and book accordingly. It really depends on how relaxed you are about taking your chances on whether or not you will arrive to find a vacancy.