The common misunderstanding about sway is, that it is something that happens when a truck passes or a wind gust exiting a tunnel, etc. Actual sway, the sway we're all concerned with while towing, is something you will have little time to do anything about unless you're on top of your game and use the trailer brakes. Left unchecked, actual sway can have you upside down in the median. So, IMHO, you really don't want to wait for that even to decide. Safe is better than sorry. I would use an effective sway deterant device. Every time. That's just me.