I only use the lines as a general guideline to keep things straight, but my ruler/ templates are what I follow & ALWAYS use the same ruler during a project, to ensure things are exactly the same. Believe me, they're not all even! If I have to change rulers (too long or short) in the middle of my project, then I double check it against the one I was using before. The only time I would rely on the mat lines is if it's a quick project w/ a lot of room for error.