I think one of the most important things in keeping a true 1/4 inch seam is to be sure the seam is properly set together as it starts into the machine straight and is held in place so that it exits the machine straight. I have observed that not keeping the pieces straight behind the machine pulls them out of line and makes the seam inconsistent. So, whether you use painters tape, a sticky pad, index card or moleskin tape, be sure it extends beyond the feed dogs.