I do have one ruler that isn't accurate. It's a 1x6". I use it for cutting 2.5" strips into squares and other small stuff. One end is about an eighth of an inch short.

I think the reason it is suggested that you not use the lines on the mat is over time, the lines will slowly wear off. I will have to agree that this does happen. I have one of the old-fashioned thick, opaque white mats. After 20 years, it have gotten to the point where some of the lines are pretty faded. I am looking at Santa bringing me a new one. Maybe even a super-sized one that fits the entire length of my cutting table! (I don't think I have been that good.)

My guess is that as long as you are consistent in how you measure, and that you double check before you slice, that will be what is important.