If the entire quilt will be made the same, you can make the rules. However, when swapping with partners or doing an IRR quilt, anything where other people are working on the same projects, something as standard as seam allowance can be very important. If you do 1/3" seams and I use 1/4" seams our blocks will not be the same. Of course, if your block is measured and meets the 12.5" requirement, I guess it doesn't matter. Uniformity is what counts.