Check your block sizes every time you start sewing or start a different fabric combination. Different fabrics can affect your seam allowance -- and sew can sitting down to sew (your position can be different so the way you eye the needle and the seam guide can change). Being off by and inch in you position can change your perspective.
Starch fabric stiff if you are going to have bias edges -- this makes it much less likely you will stretch the bias out of shape.