Funny I'm the opposite, I mostly use the mat. But whatever method I use I stick with it for the whole quilt so if one is off it all equals out. But I do often check the mat with the ruler and it always matches and my mat is maybe 10 years old. I just bought a new one since there are a couple areas where it's wearing a bit and I want to be ready if I need a new one.
One silly question, are you using new blades often? I hear dull blades can damage the mats. I'm horrible with changing out my blades, but I've only just recently gotten a few lines that won't go away.