Setting your seams does make a difference, careful pressing can help keep your block size true. I've heard experts say that you should set your seams, then pressing on the top keeps wrinkles or folds from occuring along the seams. It may seem to be a waste of time, but in the long run it makes your quilt blocks and your quilt stay true to their size and makes piecing easier. I hope I made sense with what I just said.