Be sure to use the same ruler when you cut your fabric, and always line it up the same place on the line . My ruler has a yellow line and I have to line the fabric up at the same place ( i generally have it more to the right side of the line, especially with small pieces, or that little bit off compounds itself as you work. Remember that the width of the fabric has some stretch ( not as much as bias , but more than the length), so you might be pulling your fabric just a little when you sew. And the tiniest little bit off in the first pieces compounds itself as you go along, so be sure to make the first seams match, and correct as you go, not saving any correction for the end. It always seems to me like when they say that the flap of a butterfly's wings affects weather around the world. Just a little bit off anywhere can affect the accuracy of your piecing throughout the whole piece.