On non-show quilts I use my largest square ruler - my June Taylor shapecut, on the corners. If it's slightly off, I adjust it so the difference is split across the two sides (ie: if it's out 1/4" on one side and 1/8" on the other, I just eyeball it to between the two on both sides). Once the corners are done, I add the 24" ruler up against the square ruler to make a long edge from the corner. I work this way from both corners and meet in the middle.