I only use the lines on the mat if I'm cutting something big (like setting triangles).
I use the ruler for most things but I try and remember to move my positioning. So if you have to cut 5 inch squares, don't put the squares close to the 5 inch line. Plunk the strip down a little askew, use the ruler for the 5 inch line and cut that way. Adjust it a bit every few cuts too. Then you aren't running over the same place on the mat over and over again. Sometimes I even flip my mat over!