If the seam comes out right at the 5 setting, then that's what you should use. What matters is how the seam comes out.
Was your instructor a quilter? Non-quilters do not understand that sewing a perfect 1/4" seam does not work for quilters; we need a scant 1/4" seam to allow for turn-of-the-cloth. Our patterns are based on size after seaming *and* pressing, not just the theoretical size after seaming.