I have seen this discussion on these boards before, and I went and checked my rulers against my mat and they were dead on the same. But I use both ways, depending on how big a cut I am making. If I am cutting something longer or wider than my ruler, I line it up and count off on my mat and use my ruler as my straight edge for cutting. If I am making a smaller cut, I'll use a line on my mat to be sure I have a straight edge, square up what I am cutting, and use my ruler to measure then cut. I am of the opinion that you should use whatever method works best for you. As long as the cut is straight and accurate, does it really matter what method you use?