I found that I did a better job on log cabin blocks if I took the time to square up after adding each round. I also use my 45 degree line and have been known to fudge the seam if a block is a hair smaller than needed. I think the thing that takes the most time to do but ends up saving the most time in the long run, is to mark exactly where each block begins and ends on sashing and borders. It also helps to keep the blocks in alignment if I am not using cornerstones. I do follow the old carpenter's axiom: Measure twice, cut once.