I was taught to use the ruler to measure, that being said, on large sections I do sometimes use the mat. One thing I have learned is to line up your straight edge on the line, not over not short. My cuts have been more precise since changing to this method (used cut over) Also different rulers may measure differently. No mater which one(s) you use, try to use the same brands for each project. I've had the best luck with either Creative Grids or Quilters Select. Good luck!