It is a matter of common sense and courtesy. I have to agree with the OP: I would wait the extra couple of seconds for my turn rather than cut in line. There are certain situations when I will wait my turn. For instance, if there's a family that just started processing, I'll wait and even help them out (load the stroller onto the x-ray, etc.), and then I'll cut in front of the person right behind them. It's a matter of timing. If someone has already started to load the x-ray, I can wait.