If you intend to spend the entire two weeks or most of there, then I would suggest you book ahead.
If you only intend spending a few nights there, but are determined to stay in that particular site, then I would suggest you book ahead.
If not, there are plenty of other sites in the area, and at least one of them will a space.
It has happened to us, that we would arrive at a site to be told they have no spaces. A quick look in the ASCI book will normally show you another site not too far away. The only time it has happened has been sites on the coast and generally in August.
