If you are miss cutting, say starting at 2 1/2 near you and ending at 2 1/4 on the other side...or simpley setting the ruler on the wrong line, take a mall piece of blue painters tape and attach tp the left of the line that you place on the cut side of the fabric. Then at a glance you can see if you have it straight at the beginning or the ruler is slipping.
I never cut anything that has 1/8s just because it is too hard for me to see those markings on my ruler.