I rely on both, but trust my ruler more. I have had the same Olfa mats for years, and have never cut thru them, or had to replace them.

My quilt store uses ruler lines & tilts the yardage a bit when cutting for customers. It preserves their mat.

Someone (not sure who, it was a quilting teacher) said to use the mat to line up fabric so it is straight, but to use the ruler to cut and alway leave the line on the ruler on the fabric.

I think as others have said, as long as you are consistent, and your piecing is accurate and your blocks the size they are supposed to be, it should all be fine