I find the lines on the mat to be too thick to be really accurate
David

I do on anything that has to be cut more than the width of my ruler.....how else would you do it??

never, never

OK. We are not building rockets here. The "smidge" of difference between the ruler and mat are so small I doubt if it will ruin a quilt. The difference between the ruler and mat cannot be that bad or you would have noticed it before you cut. If you cut the whole quilt the same way, they will all be the same "variance" so won't matter anyway.
Sometimes we seem to be making MOUNTAINS out of little bitty molehills. If you get this hung up on a quilt, you need to relax a bit. Really sloppy cutting is the stuff that makes the pattern pieces not fit right.
Same goes for the 1/4" seam rule. I used to do all my quilt seams with 3/8" seams, because that was the width of my pressure foot on my very old Singer. Nobody noticed. Since all the seams were the same it still fit together, just a tiny bit smaller when finished. My daughter is still using that 35 yr old quilt!

I don't use my cutting mat for anything other than cutting.

Nope, I never use the lines on the cutting mat.

I use the cutting mat lines to make sure I have a straight cut.

I use the lines as well

Couldn't agree more!

For those that say they never use the mat lines, how do you measure and cut pieces bigger than your ruler? And can you explain why you don't use your mat to measure? Out of curiousity I put a tape ruler on my mat and it measured correctly with the ruler. Isn't that about as good as you could ask for?

I was taught to use the lines on the ruler because they are straight. Where on a cutting mat they can be distorted and cause you to cut your fabric wrong. And I know that for a fact that the mats are squiggly sometimes. I have one of those Ironing board/cutting board ones and the lines on the cutting board side is all squiggly. But then again I only use if for ironing. I never cut on that cutting board side.