If you are walking from SJPdP in a group of 5 or 6, and you want to stay at the same places, YES book ahead.
Booking ahead is essential for the first night at Orisson. They work via e-mail.
This time of the year they are closed, as the Napoleon Pass is closed until at least March. But send the e-mail request once your dates are FIRM. They address each e-mail request in the order it is received...the early bird and the worm... If you want to stay together in one room ASK. They will try to accommodate you.
Also, they will eventually respond in time. However, you might not hear anything until after the holidays, like at the end of February or March. Many pilgrims are put off by this approach. But, they take their holidays at this time of the year, as there are no paying customers. It makes complete sense. Just be patient and book as soon as you have solid dates.
Orisson has become a very popular place to stop after the steepest part of the entire
Camino Frances. Even though it only takes about three-hours to climb from the village to Orisson, the short day is much appreciated. It make the next day's 18 km much easier. Plus, the views from the deck are beautiful beyond description.
Beyond Roncesvalles, I recommend you book ahead every 3 days or so, for the next 3 days. Booking.com has become very popular. Gronze.es (only in Spanish) also has a lot of information on places to stay that may not make it into Booking.com. Gronze is also very popular. Once you know your group walking pace and comfort level, you can plan and book ahead as needed.
After Easter, the 'season' has officially begun. Starting in late April, traffic picks up along all routes, but especially the
Camino Frances. In early May, there are a lot of university class groups, tour agency groups, etc. So, if you are trying to book commercial lodging (hostals, hotels, etc.) you will have competition. Also, these groups typically do not stay in albergues. This is why you must book as soon as you are CERTAIN of your dates.
If you are six women, you will do better trying to book three doubles, than six single rooms. Many small hostels do not have six single rooms...just sayin...
All the other comments above are very good. This is just my synopsis.
Hope this helps.