When you cut, do you by chance support the ruler in the middle and cut the entire length? If so, you may want to change your approach: Hold the ruler at the end, start cutting - then walk your fingers up the ruler and only cut up to the point where you are strongly supporting the fabric. Also, when you find that you are off, have you measured how much and if the difference is consistent?
It may also be how you align the ruler. Is the guideline on the edge of the fabric or off the edge?
One of the ways that I control my cuts is to cut a multiple size. Example: If I want 4 strips @ 2.5" I cut a piece that is 10" wide (4x2.5) Then I know that I can cut that piece down into my 4 subsections. I seem to have less waste when I do that.