I agree with Anemone--if this is something that you will worry about, and you're not concerned about sacrificing the flexibility that comes with walking without reservations, then you might as well reserve where possible 2-3 days in advance. More advanced notice is less important for the albergues than hostales, as you're not competing with as many vacationers.
Reservations won't be possible all of the time, and if they are you may be opting for a more expensive option (maybe 12-15 euros instead of 5-6), but if that lets you walk freely and without concern, and you can swing the expense, then I think it's a price worth paying. I wouldn't bother with a tent--added weight that you likely won't end up using.
From San Sebastián, if you follow the book stages (which are only suggested--definitely not prescribed), you'll sleep in Zarautz, Deba, Markina, and Gernika the first four nights. I'd suggest that you could feel very comfortable about getting beds without reservation in Deba's albergue de peregrinos. Reservations in private albergues might be more useful in the other stops, though.