I've been leaving my S1s plugged in and on "Quick Charge" in my practice/recording studio.
Is that the best idea, ST?
If I say, "that's fine," somebody will come along and say that quick charging will diminish the overall lifespan of the battery faster than using the slower trickle charge (and my car will last longer if I drive it slower, avoid braking, and make no turns).
If you don't need to charge the battery in a hurry, then I'd leave them plugged in, with the switch in the on or off (middle) position. It's hard to know if this will ultimately make much difference if you're not taking the S1 Pros out (running them on batteries) much. I leave mine on most of the time because I'm using them for rehearsal, or testing different scenarios with the ToneMatch mixers and various gear, or Bluetooth (downtime entertainment). Whether they are on or off, if they are plugged in and the battery needs to be charged, it will trickle-charge until it is completely charged.
Leaving them plugged in (on or off) means it's charged and ready to go when I need it.