Perhaps the 'break-in period' myth is really more to do with your wearing habits of the watch. i.e. If the watch is observed as running fast/slow in any single position for a duration of, say for example, 24 hours, that same watch may exhibit a smaller (or maybe larger, depending on how well or poorly regulated the movement is) gain/loss while actually wearing the watch, as it is then in constant motion and in different dial up/down/left/right positions throughout the day/night with the arm's motion.