I used to get wavy cuts and it was because I was not "walking" my fingers up the ruler as I cut my strips. If you only hold your ruler in one spot for the entire cut, the ruler has a tendency to shift just slightly and there will be a small wavy area where you were holding the ruler. I learned this from an instructor and I have never had that problem since. I also was instructed that you can cut WOF for sashing strips but borders should be cut LOF to prevent wavy borders. Sometimes I try it LOF if I have bought enough fabric but I do not find much difference as long as I have been careful in walking my fingers up the ruler as I cut. For the newby's, walking is like "Itsy Bitsy Spider" walking by always moving your thumb up to meet your fingers, then move your fingers forward. Another tip is to always hold your pinky finger off the edge of the ruler against the side of the ruler to steady it on the fabric. It is probably just as important that you put little dots of sand paper or Invisigrip or something like these products on the bottom of your ruler so it adheres to the fabric. These little tips saved me so much time and ruined fabric because my ruler was always sliding on the fabric when I would cut. I hope some of these tips will help you. I do not use a metal ruler only because I am fortunate enough to have the Omnigrid (and others) rulers so I do not know if these tips will help you when using the metal rulers but you might try some of them. Good luck.