The most important thing about getting in early (by July) is that you will have NO REGRETS and will MAXIMIZE your chances. It may not seem that big of a deal now, but as next year goes by and you are anxiously awaiting interview requests and acceptancesd, (when it is out of your hands) you will regret not getting it in earlier. You will certainly sleep easier for the next 6-12 months knowing you got in early. Also, many schools have said they got overwhelmed with 35+/3.7+ applications after september/october & didn't have much choice but to overlook those with less stellar numbers. If you get in early, even if your numbers aren't stellar, you have a better shot at getting an interview. That is your goal at this poiint - getting the interviews. All certainly isn't lost if you are taking the Aug MCAT, you just have extra pressure to ace the exam to compete with all of the excellent applicants that pop up that time of year.
I think everyone who has been through the process would advise you to get it together as soon as possible. Get your letters together early, have an extra if possible. Many people get delayed by 3+ months due to letters, and have ruined relationships with letter writers due to the delays. Also realize, it can take weeks for the application service (AMCAS) to accept your application after you submit it - if you wait until they are busy, it will take longer.
The point is, it is a gamble to submit late and there are too many risks and uncontrollable factors in this process as it is - do yourself a favor and eliminate this one.
Good Luck!
- one more reason to apply early. Getting your amcas app in is just the first step, then schools send secondaries that can take 10's to 100's of hours to complete. You still have to get them submitted before your are 'complete' - if you get the amcas in early, you can use the summer (which is likely an easier time of year) to work on them.