I think the best advice is from someone on the route. We just ended our Roncesvalle to Burgos today.
It is imperative that you reserve in Orison or Roncesvalle NOW (though if you get to Roncesvalle by 1 Pm, it may not be a problem). We arrived May 18th at 4:15 pm, with a reservation, and witnessed MANY people turned away at Ronsesvalle due to the beds being full. It took us 2 1/2 hours just to get registered (but It may be in part due to the time we arrived).
Since we were not registered until 6:45pm, we witnessed a lot of desperation. They even announced that ALL beds all the way to Pamplona were full.
After Pamplona, generally beds were available if it was a large albergue AND you arrived by 1 or 2 PM.
There is a really low number of beds around Villafranca and especially Ages and Atepuerco.
Many people reserved at least 2 days ahead, at all times (farther ahead if you want to get into a cheaper, private Albergue). The few times that I wanted to reserve 1 day ahead, it was very difficult to find a bed, and usually I could only find a hotel or hostel bed at a higher rate.
The
Camino Frances has been crazy busy from my May 19th departure. A more recent departee may guide you a little better. This is my 4th time and the
Camino Frances has changed. Someone who we walked with, and is walking now feels it has even changed/gotten busier since last year.
It is still a great experience, but I thinks it's good to have updated info. A few days after you start, you will have a feel of what is right for you. Buen Camino.