Well, it's a little more complicated than some of the above posts suggest.
I submitted AMCAS in June and received 6 of 10 secondaries by the end of August. Those schools didn't do anything with the secondaries until they got my MCAT scores in late Oct. The other 4 secondaries showed up in late October.
At some schools (UAB, Colorado, Nebraska, Michigan State) this was not a problem; I got interviews and was done with them by early Feb.
At other schools (UVM, OHSU) I got late interviews (March).
At the "high-ranked" rolling schools (UCSF, USC, Stanford, Mayo), I felt I was very much behind the wave of earlier applicants. I still haven't heard from Stanford; withdrew after 3 months of waiting to hear from USC. I had a 39 on the MCAT, so I don't think my score was what held up my application.
It's a numbers game. You want to make your application as visible as possible. If you are applying to your state school (except CA) August is fine. If you are a stunning applicant August is fine. If you're applying to very competitive, rolling schools, I really think you need every advantage you can get.