All (except for the first generation Asgard, for which they offered retrofitting) Schiit amps have a mute relay. They state that it is safe to leave headphones plugged in all the time. Some people still unplug their phones before turning on or off because they are paranoid.
I leave my phones plugged in all the time and I've never had a problem.
Technically, all current Schiit amps that need muting relays have them. I believe the Valhalla 2 does not, since because of how tubes power on, it is not necessary.
Outside of that specific exception, yes, RickB is correct