This is one of the reasons I have to calibrate every instrument in my lab. Even if something has a marking that says a certain measurement. Depending on the size of the instrument the precision and accuracy can go out the window.
I know, I'm a little goofy but I actually have "calibrated" my rulers and noted whether or not they agree and how much they differ. I don't necessarily get rid of a ruler because I might have an 18 1/2 in square ruler that will square 18 1/2 inches well but the markings inside are inacurate. I have found that switching brands in a project does make a difference. The lines on Omnigrid rulers are pretty fat but Creative Grid ones are very fine.