Depends exactly where you will be, but if you are walking into town or especially city rather than pueblo, I would suggest reserving for that particular day.
But if you're just going to be getting into some normal village place, it may not be necessary.
One word of warning -- that particular area is more popular than most from pilgrims who have started on the Central deviating to several villages along the Coastal (or indeed vice-versa) so that the Albergues in that area can be pretty busy.
Also, as you will be walking as a couple, there are certain choke points along that route that should be well prepared for by booking ahead -- mostly, just calling ahead in the evening or during the morning should be enough, but there are some particularly attractive places where a bed can be hard to obtain. If you're walking solo and with sufficient past experience you might get by, but married couples with shared responsibility for each other should certainly be more careful towards such things than they might need to be on some other less "touristy" routes towards Santiago.
There are stretches along the Coastal where you can still more or less "wing it", but there are some locations where only the more experienced pilgrims might be able to get a sleeping spot for one person alone, and not necessarily a bed.
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Having said that, you need not necessarily book so far ahead -- cancellations are common along that Way -- I'd say at Porto itself reserve, then out from there 2-3 days in advance ; until you get a "feel" for things, speaking with other pilgrims especially, and from there try and get a sense of places where you need to reserve more than a few hours ahead ; there are a couple of places where days ahead is safer.