Unfortunately, AMCAS says they won't evaluate your application until all materials are in, including the last MCAT. Because of this, I don't think it matters if you submit your app before the August score report because it will just sit in a file until the score comes in. I suggest doing it whenever you have more time on your hands for transcript requests, evaluation letters, etc. If you put it off make sure you apply immediately after the score report.
This happened to me last year. By the time the MCAT was scored and the application service finished my app (6-8 wks?), the schools I applied to had either 0,1, or 2 interview days left! And don't forget the schools shut down for a week or two during Christmas break.