Being an 1/4" off here and there, can come around and bite you later on. If 12 blocks are 1/4" bigger than they should be, the row will be 3" too long. So your measurements for the borders in the patterns would be inaccurate. If your borders are pieced, this will make a major problem for you, and that is a lot of extra fabric to try and ease in.
The more pieces going into a block, the more opportunity for variances, and the more important it is to check for accuracy. A 12" block with 9 pieces won't vary as much as a 12" block with 36. If you find your piecing and cutting is fairly accurate, you may just want to spot check your work.
Using different brands of rulers, or the grid on your mat can make for variances too. There is a difference in some brands of rulers :wink: You may want to compare yours, as sometimes that may be what is causing a variance in your piecing/cutting.
I tossed out one ruler because when I cut out 4 in blocks, they were all just off far enough, that a scant 1/4" seam still made them too short when pieced into a 9 patch. I cut them with different ruler and they were spot on. I then double checked them with another ruler and got the same perfect measurement. Had I of used my mat for part of this process, I would have ended up with a slightly different measurement also.... I wondered if the machine in the factory was a bit off and in need of an adjustment that day :roll: and that is what caused that ruler to be off...