My experience after three Camino's (2 spring and 1 fall)
1) Orisson-goes without saying
2) Roncevalles if you are walking from SJPdP in one day and expect to arrive late (depending on time of year.)
3) Zubiri was completo in all three Camino's-I suspect because people under-estimate the difficulty of the day and are ready for a stop.
4) Towns after or before a long stretch (Los Arocs, Carrion de los Condes..) This is particularly true if you are walking a long day and will not arrive until late in the afternoon.
5) If you are walking out of a major city during the May 1st Spanish holiday. I was walking out of Burgos on the Monday of that week and was only able to find a private room in Hontanas 34km from Burgos by calling ahead. Every town in between was full and the albergues in Hontanas turned away pilgrims all afternoon. It was another 9km to Castrojeriz. People were taxing back to Burgos.
6) Staying on
Brierley stages Sarria on....while I can't speak to the accommodations for this section as I stay off stage, I can tell you that the path will be more crowded if you stay in a stage town. If you stay in a smaller town you will be walking with fewer people.
There are so many towns and albergues and you will quickly get a feel for the numbers of pilgrims related to accommodations. Even when busy I wouldn't book more that a day or two ahead with the exception of the first few nights. You want to leave yourself flexibility to change your distances or stay with other pilgrims. If you do make reservations and change your mind, please be courteous and notify the albergue as soon as possible. There were pilgrims making reservations in multiple cities to give themselves flexibility. I won't even describe how wrong I felt this was. Buen Camino!