Sorry about the glare! but as you can see, it's about 1/8th inch off where the dark green print is under the ruler. That inaccuracy will be hidden in the seam allowance, but I like a clean edge when I am joining blocks, or else distortion can build in.
Here is the fix I've come up with for these little glitches:
You will need a piece of freezer paper and your 'paper scissors,' because I *know* you would not cut paper with your fabric scissors, right? Cut a strip or two the length of the block and about 3/4" wide. It need not be neat.
Put the block face down on the ironing board and lay the freezer paper strip along the edge, overlapping the edge but covering the area where you have come up short. Press it. You can see that I had two places where the measurement was slight shy.
Peel the whole shebang up and go to your cutting board.
Put the ruler over your block and square it up, freezer paper and all; then cut away the excess. You have now created a clean edge and have an accurate block.