As long as the HST will fit with any other components of the quilt, size doesn't matter. If the quilt will be all HST, do whatever you want. If it says to start with 5 inch squares and trim them down once they are sewn, I would say they are instructing you to use the method of drawing a diagonal line on the back and sewing on each side. That yields a HST 1 inch smaller than the original square.