Peter, I prefer doing my clay renders using the #7 preset to keep the shadows from blocking up. the few times I tried to render with the AO preset I got much denser shadows that I wanted.
Here's the most recent version of the built-ins. Quite reduced from the original version but that's alright.
http://farm9.staticflickr.com/8094/8504207294_ed87deaa4a_z.jpg