If by "postbac" you mean "I have a low GPA and am trying to do something about it" the short answer is "finish your GPA redemption, get an above average MCAT, THEN apply".
If by "postbac" you mean "I have a liberal arts undergrad 4.0 and got into Bryn Mawr" then the short answer is "Bryn Mawr has linkage so you're probably applying during your postbac".
If by "postbac" you mean "My GPA is > 3.6 and I'm taking prereqs and/or fresh science coursework wherever I can and hoping to take the MCAT in the spring" then the short answer is "apply when you have all your numerical assets (GPA&MCAT) in place, no later than mid-July".
In other words:
- high GPA? apply during postbac
- low GPA? apply after postbac