When I was in the learning process, one of the first classes I took, my sister in law and I were cutting and using the lines on the mat. The girl who taught the class told us that the lines were not accurate, since that day I do not cut any pieces for a quilt using the lines on the mat, but use my ruler. Now that does not mean that things always line up perfectly, they do not at times. That I blame on my 1/4 inch foot on my machine. I have a time of positioning it just right and a line of tape to get the 1/4 inch. I have found when I start out with something new, as quilting was, if I am shown the correct way I keep on doing it. Just my thoughts. I do use the lines when I want a rough bit of accuracy for some things.