I don't use my mat for measuring/lining up. Ruler only. I had been using a 6" X 24" EasyRule forever. Went and bought a 6" X 12" Creative Grid a week ago and Monday went back for the 6" X 24". These rulers come with the grippy things etched into the ruler-kind of like etched glass. I checked both old and new rulers and switched to the new mid-project. Top came out square.
As far as the mat goes, I have an Omnigrid. Depending on my fabric color, I will flip the mat over to the grey side to see the fabric thru the ruler easier.