I was also told not to use the lines on mat and to check out the rulers against each other before using more than one ruler on the project.
After saying that, I do use the lines on the mat for one thing. When I am cutting a wide, big strip, I first measure how much I want with my ruler, straighten up my material on the mat so that it is squared up right and then use the lines on the mat to help orient my ruler so that I will cut straight up and down instead of a little bit diagonal.