I think the most important thing is to square up the blocks as you sew. If I'm working on say a 12" finished block I square that up to 12 1/2" before adding it to another block or sashing. Once all blocks are sewn together I will take 3 measurements each way (middle, each side) (horizonal, vertical) to ensure they are correct before adding the first border. If your blocks are coming up SHORT, I fudge that with the sashing or the next block ( of course making sure the next block is correct first).