To me post fader monitor wedges on stage, or in an orchestra pit, are a little strange. But this is exactly what we do with large musicals with lots of lavs. I often mix 20+ vocal mics from VCA's at FOH and I want the musical directors wedge to be a representation of the FOH mix. So, that when an actor walks offstage and I mute a VCA, it takes it out of the monitors too. That's kind of an imperative.
Usually I don't use the mute groups for mixing but instead for utilitarian stuff, like:
When we go to break and I want only the SM's mic on
Muting only multitrack playback
Leaving only specific groups of monitors on for rehearsing