I use both, nothing wrong with lining up your fabric on the mat ruler lines before you cut the fabric. I have several slotted rulers and I use them all.. if you use the ruler on the mat first you keep from cutting the wrong sizes and wasting fabric that way because you tend to forget or not notice the size of the strips you are cutting.. believe me no one wants to waste fabric, and I have made many mistakes by not paying attention to the mat ruler first.. good luck.. follow your own way that makes you feel comfortable