I have never walked the Norte in August, but since it is high tourist season, tourists will be competing with peregrinos for beds. And no surprise that the businesses prefer tourists with their fatter wallets. If you are going to be walking in August, I would definitely reserve, and it would also be a good idea to try to avoid the main, standard stage ends.
I think its wise to book for Zaruatz. It has fairly limited pilgrim options I think.
I followed my guide book and planned to stay in the youth hostel, but they dont take pilgrims anymore. I then walked back to the start of town and stayed in the Zaruatz hostel. I took the last bed there ... it was 22 euros, so pricey.