In the Mat Lines vs Rulers controversy, you might want to check ALL of your rulers AND mats for accuracy. I accidentally discovered the first discrepancy in my rulers when I was stacking them neatly one day, fiddling around while on the phone. As I hung up from the call, I happened to notice that the ruler on the top did not match the ruler underneath. Which did not match the mat.
And I buy good rulers, too. I have an omni mat and set of rulers. Almost every one of them is different.
I finally went to a store where architectural supplies were sold, and found that only one of my rulers was accurate to the distance of 12 inches. That is the ONLY one that I use to measure cuts anymore. Some of my rulers are almost a full 1/4 of an inch off over 12 inches. My mat is pretty darn close, though.