It is quite easy to block your blocks and make them a perfect 12.5
Go out and get yourself a cork bulletin board. Take your 12 1/2" square ruler and a sharpie permanent marker and trace the 12 1/2 square onto the cork board.
Take your finished block and pin it to the cork board, pinning opposite corners first then the sides, use as many pins as needed to stretch out that block to meet the drawn lines. Get a spray misting bottle with plain water. spritz that block until it is good and damp then go on and do something else for several hours or overnight allowing the pinned block to dry in place. Remove pins and like magic you will have a perfectly squared up 12.5" block.
Blocking can also be used to square up a completed quilt as well.