I agree with Kalamaquilts. If seam allowances are accurate and are pressed without wrinkles, you shouldn't have to square up your block. Before starting any quilting project, I do a seam test as she described. Then throughout the piecing process I measure finished pieces to make sure I'm still sewing accurately. Should I have a problem then I can correct it before I get too far along. If you do this, your seams should nest together perfectly without a lot of pinning. However, that doesn't help you with your current dilemma. So, I suggest pin your seams as some of the other ladies suggested. It may be that since you have squared your block the seams may not match up at all. Wishing you good luck and happy quilting. I might suggest Harriet Hargrave's "Quilters Academy" books. They explain the fundamentals of piecing. She has published four books so far. The first one dealing with basic piecing of squares and rectangles and each dealing with progessively difficult piecing techniques, I had always preferred applique because I could never get my piecing to go together well (same as you are experiencing now), but since I bought and followed her techniques I can now piece with confidence, and it is so exciting to get your seams to match up without stress.