Hi,
The first time (April-May) I only pre-booked at SJPDP and Santiago and then booked online (when I could) an hour or so before we were arriving in a town. That worked out great. We only had a problem in Palas de Rei where we had to search for an empty room which we found.
The second time (late May to July 2) I pre-booked everything I could because 1) I love to plan and I had a much better sense of how long we could walk, etc. and 2) I was under the impression that there were going to be a lot more people in June (there were not).
In hindsight, I wish I had not pre-booked so far in advance because I did not take into consideration weather (being forced to get someplace in a thunderstorm or in sweltering heat), needing a rest, or wanting to go further (all the things everyone discusses in this forum that I wished I had listened to
).
I pre-booked accommodations for another travel companion who ended up walking in five hours what it took us two days to do. After Zubiri, I cancelled all of his reservations and he did just fine finding places to stay each day.
Having said that, based on the crowds we saw after Sarria (there were A LOT more people after Portomarin in June than in May), if I were you, I would have a general idea of where you want to stay after Sarria and book ahead when you have a sense of when you will get there and how far you are walking each day. Hope that helps! Buen Camino.
EDIT: There were a lot of school groups in June starting in Sarria - that might not be the case in September.