AFAIK, “Start/Stop” will adjust it at the Start/Stop of the movie while “Always” will adjust it any time even if the refresh rate changes during a movie.
I believe “Start/Stop” is the best balance.
I think the only practical difference it makes is if you’re in mid-playback and you hit the Back or Home button to get into the settings while the video is still running.
If, for example, your GUI normally operates at 60Hz but the film is 24fps, and the setting is “Always”, switching between the GUI/menus and the movie requires a refresh-rate change each time. If the setting is “On start/stop” then it only goes back to 60Hz when you actually stop the film - going into the GUI while the film is playing gives you GUI at 24Hz.