Like some others, my wife and I are planning on doing some biking in France, probably Loire Valley and/or Burgundy. We're planning on late May to mid June (not all of that biking). When we were a bit younger, we toured much of the U.S., Austria (along the Danube) and Ireland, and just rode as far as we liked, then searched for TI and found a B&B, or perhaps a hotel. We almost never reserved ahead, except when entering super-touristy cities. But we ride shorter distances now, which leads to my question: Are CHs or B&Bs common enough in small villages for that plan to work? Or would we run a risk of sleeping in some abandoned doorway, unless we make reservations at least a day in advance?
I think it is a good idea to call ahead the morning of. If you'd like to stay in a very small town, advanced reservations, or an alternative plan, is in order that time of year. Here's my experience cycling in Burgundy in mid April. At that time vacancies abound. (much more than in June.) But we went through plenty small towns that had NO hotels or B&Bs. That level of planning is just due diligence. Also, we got so used to just casually "winging it" that we missed out on a place we'd wanted to spend the night. Not because they were full, but because they weren't home when we came by, and we did not want to wait around while the sun set. So we peddled to the next town and found a room there. We should have called in advance. There's one amazingly charming village in Burgundy that has but one creaky old hotel with but ten rooms. In April we were the only guests, and we dined with the landlady. In June, it could be full. In a larger town you could probably be sure to find something. So it varies. If your route includes a small town not far from a bigger one, you have options there.