You can let them do whatever you like! I usually let them grow until I can bend them down to the growing medium. But if they run wild that's fine too. Then when you repot you can take whatever long roots you have and pot them up. And to clarify, all of the roots are the same, looking for the same thing, moisture. They also can photosenthesize, which is why the ones exposed to light turn green when wet. Good luck! Plant looks great!