I'm pretty sure that pressing is your culprit. As quiltstrings mentioned - press your strips with them lying vertically on the ironing board. You may even want to draw a straight line on the board cover to keep you focused. Remember to set your seams first - press them the way that they were sewn - and then press the seam allowance in the direction you want them. Even lengthwise strips have a tendency to "rainbow" if you aren't paying attention. Alternating the direction of your strips will help as well. I've been through this- and having to re-square constantly while cutting segments - but I don't anymore!