I participate in block exchanges and was always frustrated that MY blocks weren't coming out to the right size, couldn't really get them square, ect. I finally decided to select blocks that I could piece a little large and use the 12 1/2" square ruler.
Since then I am happier, still struggle to get the exchange blocks to go together, but have learned to add sashing, accept the blocks as they were intended, a gracious act of generosity and community and put them together and enjoy my quilts.