There are so many variables on finished block sizes, I am glad I am not a perfectionist. I can appreciate the work that goes into a perfect quilt, but for my own quilts done is better than perfect.
I do try to determine where to stitch to have the correct sized block with each project, but I use a mix of washed and unwashed fabrics in my quilts (I do not wash precuts), some of the fabric may be starched, I use different types of fabrics, regular quilting cottons and batiks in the same project and sometimes I commit the cardinal sin of using more than one machine to piece my quilt.