I think that using the lines on the mat as a straight edge with your ruler to make that first straight edge on the left side of your piece of fabric (if you are right handed) is completely different from using the spaces between the lines on the mat as measuring units. The method I was taught in a quilt class was to use the lines on the mat to get your fat quarter, etc. aligned with the mat. Then line up the ruler with the mat line for that first cut only. From there it was ruler only to measure and cut the strips. This method requires that the ruler is placed on the piece being cut away from the whole. I agree with whomever pointed out that placing the ruler on the piece you are cutting prevents ruining the piece if your rotary cutter should veer away from the ruler.