You are getting a lot of great advice here. I had the same frustration. I now use a 50wt thread on top and 60 wt on the bottom to piece, both by Superior Thread which isn't necessarily relevant, and also cut my fabric allowing an extra thread on the fabric edge if my finished pieces aren't coming out 100% true to size. I use my 1/4 " foot as a guide, but move my needle position and stitch a test square as advised previously and check the end result to make sure I'm getting the finished product the size I'm looking for. That is the only way to check whether or not your seams are accurate. I am happy to say my piecing accuracy has improved 100% since making these changes.