I do use the lines on the mat and on the ruler. :D

matt lines aligned with the ruler lines to cut
how else can one be on the LINE and straight?

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.

I use the lines on my mat just as guidelines...I trust my rulers a lot more!

I do often enough to say I do. I think you get a more accurate cut that way, but sometimes I don't have to use the cutting mat's lines.

I, too, use both methods at times, but mats are not always accurate, so I teach my students to use a ruler and not rely on the mat markings. Also be aware that you CAN indeed use a ruler when the cut is wider that your ruler...just put 2 rulers side-by-side! I do that all the time for wide borders, especially if your mat is well worn or not terribly accurate.

I do both!

I never used to use the lines on my cutting mat; always thought the rulers were more precise. Until I made my bow tucks bag, then it dawned on me "why and I struggling so hard with this" butting up one ruler to another to get the width/length I needed. Now I use the lines in conjunction with my rulers but use the rulers more than the mat. :-)

I do this all the time.

exactly what she said. They don't always match and it doesn't make a difference which one you use as long as it's the same through out your project.