Classes you take after you graduate will count towards post-bacc, if you take them before you graduate they'll fall into the 5th year. They can be the same classes, it just depends on whether you have a diploma already when you take them or not.
One advantage to doing it as a post-bacc would be that you would have another line of good grades. A reviewer looking at your AMCAS would either see:
Freshman: 3.0
Soph: 3.2
Junior: 3.4
Senior: 3.8
or
Freshman: 3.0
Soph: 3.2
Junior: 3.4
Senior: 3.8
Post-bacc 3.8