We all do this. I've been building wood bows for 25 years and still have to constantly check limb thickness as I tiller from a vise. Once I get to my bowyer's bench it isn't as prevalent. If your vice is mounted on your work table generally the side closest to the table will be the thick side. And when you flip the bow to work on the other limb you do the same but it is the opposite side of the first so you get a propeller twist.