A recent thread on rulers reminded me that if you are shopping for a new rotary cutting ruler, look carefully at the lines on it. Some rulers have lines that are rather thick. That can make a huge difference in how accurately your blocks turn out.
I had a beginning quilting student and no matter how hard she tried, she could not get her blocks to turn out the size they were supposed to. We finally realized that the ruler she was using had lines that were about 1/16" of an inch wide. When she lined up the fabric on the edge of that line, each of her pieces was coming out 1/16" short. That might not seem like a lot, but that adds up to 1/8" for each seam, and if there were 4 seams across the block, that would add up to 1/2" short!!!!
Moral of the story - if accuracy is important to you, when shopping for a new ruler, look for one with the thinnest lines possible. And always use the same ruler to cut all the pieces for a project.