Well, not bucking during tourist season is always a risk. Some people don't mind it and they consider it a part of the whole experience and others prefer not to be stranded looking foe lodging in an unknown city. Usually (at least in my experience) booking in advance allowed me to get the type of room I want in the pricerange that suits me best. Tis may be something you may want to consider if you have a really tight budget. On the other hand many backpackers do not book anything untill just a day or two in advence (over the internet let's say) or head for the tourist information office when they arrive in the city to find available beds and book them then. It entirely depends on you and your traveling style.
To judge the availability of hostels and how much of them are actually booked you can always check it through www.hostelz.com
or any other webpage to get an idea.
I guess if you do not want to remain flexible you can always just book the first night in advance - this way if you have to cancel it will be cheaper (if you do it over 72hrs in advance there usually is no penalty or just a few dollar booking fee). If you don't like the place you will at least have a single night with accommodations and then you can go to different hostel... or if you do like it then you can make arrangements to stay there longer once you wake up the next morning.
Welcome to Tpunk and I hope this helps