If you are referring to a traditional masters degree, then postbac undergrad classes, which have the potential to raise your cumulative undergrad GPA, would have more impact on adcomm decision-making (as would delaying graduation for another year, provided you earn excellent grades either way). A Special Masters Program or similar would be an exception. However neither approach will help you much if you aren't getting an A to A- average. Why not start aiming this high now, while you have yet to graduate, so you have less GPA repair to do?