You loose accuracy if you use the lines on your mat. If you are off 1/8" on each square, and that is easy to do, especially if you have several pieces in each block, then if you have 4-8 blocks in a row, you would be off 1/2 to 1 inch. That makes a lot of difference.
Sue

I use the lines of my cutting mat to line up my fabric but use the rulers
to cut. My cutting mat is metric (LOL) but still in good shape.

I use the lines to help line up the fabric, but use the rulers for cutting.

I've seen Eleanor Burns (in videos) use the lines on the mat for cutting.

I never use the cutting mat lines when cutting.
Only use my rulers and I have several sizes,this is the way i first learned and the way I have always done it. :D

I don''t use my mat because it is warped - too many times used as a placemat I guess. Since it never goes off the dining room table LOL!
I use my rulers exclusively. Esp my June Taylor shape cut. I love that ruler!

Me too!! I have never understood putting the ruler on a 2 inch wide section of a yard cut of fabric. It made a lot more sense to me to leave the 2 inch part free lined up with the mat and cover the rest to cut. I tried doing it another way that I saw on a video - just couldn't make it work for me. I have an Olfa mat and have never had trouble with the measurements not being accurate!

If you use the cutting lines on the mat, soon you will have permanent grooves and your mat on the measurement lines and it will become useless -- I turn the mat over and don't use any lines at all but use the ruler for measurements. Also make sure each ruler you use are synced. Not all rulers are equal, and if you swap between ruler, make sure each measures the same. Hold them together and check to see if the lines match up - surprise, many don't especially if you have different brands.

CrashNQuilt is sooo right

Never use those lines

I quit using the lines on the mat after taking a class years ago but after watching the Quilty video I think I'll give the mat a try again.