Depends on the grade in the class. If it is a grade holding you back, retake the class. That is basically what a postbac is, retaking basic science courses. If the grade is decent, i.e. B and up, then take higher level courses to make yourself stand out. Don't get bogged down retaking every class with a B to get get an A because that is just a waste of your time. Try and get A's in those upper level courses, it looks better that way.