Always make a reservation. One of the good things about making reservations is that you may learn that the place you were planning to stay was closed, under repair, etc. I almost learned this the hard way, while in Galicia, I wanted to stay at the Casa Grande de Cornide in the outskirts of Santiago de Compostela, I thought that since I was going in December the low season, I would not need a reservation.
Fortunately, someone on this board, adviced me to make reservations. I was glad I did, for when I called to make my reservation the owner told me that they were closed from 20 of December until January 10th!
Lesson learned! Don't assume anything when you travel, always ask first.