If you separate your blocks with sashing you can fudge the difference out and make it less noticeable when blocks aren't the same size. I used to have that problem all the time because my 1/4 foot wasn't correct. If I'm doing a 12 1/2" block I will lay the block on my cutting mat within the 12 1/2" square, then lay my sashing down on top where the true edge should be. Pin together and sew, but first making sure I've got enough of the block in the seam allowance.