I have always used just the markings on my ruler or rulers, and make sure that my rulers are the same brand. I have heard stories about people using two different brands of rulers for a cutting project and then end up with them not being the exact same measurement and the quilt top wasn't right. I think the same goes for using a mat with lines for measurements. I may use the lines to make sure that the fabric is laying straight, but as far as cutting using the mat instead of the ruler for measurements, no, I don't do that. My first mat was a long time ago, and it didn't have lines on it, so I didn't get used to using them to begin with. I guess I'm telling my age here, old, but I believe that for me anyway, I'll stick with using the rulers as my measurement in cutting fabric. :)