Agree with all above. I never use the lines on the mat for measuring to cut, only to make first cut, after that only the same brand rulers throughout the project. Actually I use the backside of my mats as they were given to me by a LQS as they bought way new fancy ones. Two old greenies cover my sewing and cutting table.

I agree with keeping the bulk of fabric to one's right as they cut...however in my experience...I have to regularly check the accuracy of the left edge to see it remains true and does not slant a tiny bit. I really like to stay on grain of fabric and sometimes in my haste I stray...