I teach to always have the ruler on the 'good' piece cutting into the extra. So often the cutter can slip just a tad and cut into the fabric. If the piece you want is under the ruler, then your slip will be into the extra. Your line on the ruler should be on top of the line of the fabric, not to the left or right of the edge. Also, make sure you are holding the cutter straight up and down. Not only is this more accurate, but it is safer. Until you get your cutting more accurate, cut slightly bigger and trim the blocks down (if the pattern allows for this).