I checked this link "Cutting Fabric from the First Cut" and this is how I cut first cut. Then I use my ruler to cut other cuts keeping the bottom folded edge lined up with a line on the mat. One important thing that was mentioned in this post is CONSISTANCY meaning using the same ruler throughout the project. This is aLso true if you use a mat as your cutting tool. I do know understand how you can always use the mat for cutting lines since the fabric edge does not always stay on a mat line. Do you keep lining up your fabric to the mat and if so do you not take chance of the edge not staying even as you move the fabric.? I just find it easier to use my ruler.