Pressure on the ruler is the key. All of the starching, trimming and alignment tricks are defeated if pressure isn't maintained from one end of the cut to the other. As one pushes forward with the rotary blade, the blade may push the fabric from under the cutting guide, thus causing a curve. A dull blade, or one which no longer rolls smoothly because of age or dust build-up can make it worse too. David, is your cutting table low enough so that you can sort of lean onto the ruler to keep steady pressure and do you walk that steady pressure evenly as you roll the blade? It gets back to the angle of the dangle thingie.