If you guys are open to stay in small private inns (which are very cheap) in case albergues are full, you should not have any issues at all.
I'd highly recommend booking the first day of walk, maybe the second. From there on, it is better to decide on the go, or maximum a day before - weather, you adaptation to the walk, the food, everything can affect.
I had pre-booked a place on the day we went up Alto da Portela (the 'killer stage' on the Portuguese Central route), believing we would be destroyed when we got there. It happened that the climb pumped us up SO MUCH we ended up walking almost 15km beyond what we planned. I cancelled my booking (it was a free-cancelation booking made online) and stopped at a Casa rural later on the trail.
After a few days on the road, and with a better understanding of my body and the trail, my routine became to arrive at accomodation, wash, eat and then evaluate how much I wanted to walk next day. Then i'd look online for a free-cancelation booking somewhere around that region and book it. Next day, depending on how the walk was going, I would amend it or not.
Just a reminder: if you do that, ALWAYS cancel your booking if you change your mind, so another pilgrim can get that spot and the owners do not lose business.