I've found that what makes a fabric "bow" is not so much the cutting or sewing, as the pressing. I drew a straight line on my cutting board so when I press strip sets, I can be sure the seam stays perfectly straight. Also, I have found that if it is possible to do with the pattern you are working with, I piece just half the width of fabric strips, not the full width. They seem to be easier to handle and don't stretch as much. Easier to keep pressed straight, too. Sometimes this is not possible because of your pattern, or because you don't have enough fabric, but if you can do it this way, it's worth a try!