To book or not to book

We regularly go to France in September and have never booked a site in advance.  Only once have we had difficulty: in Challons-en-Champagne.  We arrived in the late afternoon and found that the site was so full that we had to be squeezed in on a patch behind an outbuilding which was not intended to be used as a pitch.  The following day, further South, we arrived at our destination and got a perfect pitch on a site which was 80% empty.  This year we are considering crossing, from Poole to Cherbourg, on 25th August but wonder if we need to book sites for our journey South.  Has anyone had experience of Normandie and the Loire towards the end of August?  If anyone can tell us if we need to book that would be a great help.  Thank you.