If I remember correctly, after you send your application in May (the earliest you can send it) you are not available to make any changes, unitil late November or December (I just went through the process and I don't remember...). By that time, if you make everything early, your application have been review and interviews offer. So taking classes to improve your GPA will be just in case you don't hear from school by December 1st... AADSAS takes all your classes and average them so if you got a C and take the class again and get an A your GPA will move just slightly, I don't think is that worth it!!!
Good Luck