Reserving a bed depends on a few factors... When will you be walking? Will you be alone? What type of sleeping accommodations are you looking for? Will you be carrying your pack or having it transported?
I calculated that there were probably around 60 walkers on each stage so the demand is much less significant than the hundreds walking the Camino in Spain. The only time I had a hard time getting a bed was the Saturday night of a long holiday weekend when everything was booked. My Australian friends often had trouble because they needed three beds.
Normally (non-holiday weekends), you only need to book the night before or even the morning of the same day. After Cahors, the baggage carrier service (Transports Claudine) requests that they be notified at least two days in advance, if possible. Once I figured out that 20-25 km was my ideal distance, I calculated where I would like to stay and pre-booked 3-5 days in advance.
I always tried to find gites that were demi-pension and I think these fill up faster. I know several people who never booked ahead, they just kept walking until they found something, but they were capable of walking 50 kms, if necessary. Most of the municipal gites (no meals) I stayed in had empty beds, even in May, which is the most popular month. There also seems to be a growing number of people who reserve and don't show up (a big frustration for the gite owners). If you're willing to chance it, even gites that were "full" ended up having beds available.
A couple of notes:
I were to do it again, I would leave mid-May so more flowers would be in bloom and more livestock in the fields. That is also after the 2 early May holidays.
The meals you get with demi-pension are well worth the price (usually an additional 12 euros for an all-you-can-eat dinner), even for a non-drinking vegetarian. I once got a salad in a café that cost me 15 euros. I know some of the meals I had would have cost at least $50 in an L.A. restaurant.