I use the vertical lines on the cutting mat if I'm cutting strips. I use an O-lip-fa ruler that firmly hits the bottom edge of the mat to stay straight, and I make sure the lines on the ruler match up with lines on the mat. I usually have to move the fabric slightly to keep the edge right on the line before my next cut. If I'm cutting or trimming small squares, I prefer to use a square grid ruler so that I can easily line up a horizontal and vertical edge at the same time, and in that case, I ignore the lines on the mat.