Actually, going from left to right and back to front (both measured as you stand in front of the machine) are the ways of least resistance. That's not to say that you can't go in circles, spirals, or right to left, front to back - but for maximum speed, sewing more or less straight lines, I was taught that you should go from left to right or back to front.
I believe this has to do with the way the machine is threaded. The thread comes from the right of the machine through the tension assembly, and goes front to back through the needle,then into the fabric. As you move the longarm head, you have tension from the thread that has already been sewn into the fabric and from the tension assembly. The thread going through the needle is pulled both ways and can snap at the needle if the tension is too severe. (Remember that the thread goes through the needle a gazillion times when making each stitch, so it can fray pretty quickly if it's being pulled too hard against the needle.) When you sew left to right, or front to back, the tension points (fabric already sewn and tension assembly) are on the same side of the needle, and that causes more thread breakage - but only at higher speeds, which is what you would do if laying a baseline for lining up your top for floating, for instance.
This same principle applies to free motion on the domestic sewing machine. If you move too fast in a direction that pulls your thread against the needle, you will get thread breakage.