Very often, it is your pressing. I too, am guilty of moving the iron instead of just setting it on the seam. Here is a little trick that I have used: Take the largest (or most wonky) block. Lay it on your ironing board and pin it down at the correct size (You can mark the size using two rulers butted together as was previously stated). This may mean stretching it at some places and pushing it together at others. Now dampen it thoroughly and leave it overnight. See if it is still as wonky in the morning. Dampening the fabric allows the stretched fibers to go back closer to their original size.
Try a couple before you start cutting things down.
And next time, square up after every round. Takes more time initially but ends up saving frustration later.