I took a rotary cutting class/demo at fabric shop decades ago when I first started quilting. The woman was a rep from Omnigrid I think. She said over an over the first thing that is important is be sure the ruler line is completely on the fabric before cutting. She had examples of sewn squares with the line on the fabric and sample of the line off the fabric. Big difference in the finished size of each block.The rulers then had thicker lines. I like Quilter's Select Rulers the best, they have thin lines. Creative Grids for the sizes QS does not have.