You can book months ahead with Camping Cheques if you wish - and usually the site will request a deposit - which is often a couple of cheques. Others on another forum have already booked pitches for next year. If you hold a 'Gold Card' (a Camping Cheque electronic version of the paper cheques) then you don't have to pay this advance deposit.
You can also ring ahead to reserve a pitch a few days in advance - and if you're on a Camping Cheque site already, then reception staff will sometimes help out with this. The Camping Cheque guide which comes with the cheques suggests sites on which it may be advisable to book ahead - and usually suggests dates when this may apply.
However, we've been using Camping Cheques for 12 years and have NEVER booked ahead, or even phoned to see if pitches were available, and have NEVER not found a pitch. Val de Cantobre is one of the most popular sites, but we've found pitches there with no problem. We always travel in late May/early June - and have even found sites to be almost empty. This year we used Touring Cheques for the first time, and at Bastide en Ardeche (one of their sites) in June this year we were one of only 10 occupied pitches on a site with over 200 pitches.
We also use an ACSI card - and I don't think you can book ahead using this - but others may prove me wrong.
As these discount schemes are all meant to be used in low season, and acceptance dates set by the sites themselves, then this should mean that they expect to have pitches available.