This question has been answered time and time again, but there will always be disagreement. The answer that is tailored to YOU will depend on other aspects of your application. Here's how it breaks down:
if you have a pretty decent GPA (3.5+), taking the August MCAT will not disadvantage you. This is because your GPA is good enough to get the school's attention, and they'll invite you for an interview once they get your MCAT scores (if you score well). Most schools wait to send the majority of their invites out after the august scores are released. (this is the cause of much unnecessary anxiety on SDN, as april takers are waiting to get interviews but they are trickling in slowly)
If you have a crappy GPA, as I did, you should NOT take the August MCAT during the year of your application. What I did was take the August 2004 MCAT and applied for the 2006 cycle. I didn't want my GPA to be the first thing schools see, without my MCAT. They would surely reject me on the spot. I knew I would score well on the MCAT, so I wanted that number on my application to balance out the GPA.
bottom line is, really early in the cycle (september-november), schools interview the superstars-- those with 4.0s and 35+ MCAT because these people are gonna get interview no matter when they apply. But the bulk of the interview spots are saved for when the school has seen most of its applicant pool (after august scores are out). If you take the August MCAT, just don't freak out about getting late interviews. A friend of mine in the same situation (3.5/35AugustMCAT) racked up 6+ interviews in the months of january/february. She even got a interview just last week from SLU.
Just have a lot of patience.
Good luck