Always book 2 to 3 days before traveling, never had a problem but we don,t travel peak times. It is rumoured that if you don at t have a return ticket booked it could invalidate your travel and health insurance, dunno if this is true or perhaps you have more chance of being gassed!!!!
Don't forget to leave a review of the French and other European campsites you have visited!
If you are travelling peak times, it's always best to book well in advance, costs rise and you'll probably find a lack of availability especially if you want to book accommodation. Out of season it doesnt matter as much as there are normally lots of cabins that you can book once boarded at a reduced rate but believe me during busier times you may struggle.