It depends when, and where you're going Billy. If it's high season, or an event weekend, they could truly be full. Make a phone call if you can. The Let's Go book series have the most absolutely up to date contact info, way better than lonely planet or the other guides for hostels, and is well worth the investment.
Also try the non HI hostels. Yes HI hostels are consistent - but they can be consistent in the "my, this place is so generic it could be anywhere", and "it's filled with junior high/high school kids on a school field trip". The HI hostels are OK, but my best experiences were at non HI ones.
The only place I had troubles booking night of was London, in the high season. All the hostels were full, but after calling B&B's for an hour form the train station, I finally found a bed. Everywhere else (15 countries), was OK.
If you have any question let me know. I've done the Hosteling/backpacking thing several times for several months.