To avoid the dog-leg in cutting strips, when cutting from selvage-to-selvage, fold the fabric in half and/or then quarters. Place your ruler near your belly on the straight edge of your fabric fold. Make sure the fold at the other end of your ruler is also parallel on another line. Continue to adjust the fold away from your body to get the edge on the same line.
If you only have one fold, then make sure the ruler is even along your only fold. Hope this helps. Suzanne