Anyone who has done live work for any time has heard this. "We'll just do monitors first and then the house." The problem is... the house is gonna be on eventually, and you do hear it on stage.
I recently mixed a show with a known, 20+ years experienced act. They are a seemingly simple 4 piece that under normal circumstances should take all of 20-30 mins to get everything happy for all. They are one of those classic stories where someone somewhere told them to check monitors only first, and that's the way they roll. So after at least 45-60 mins of "monitors only", they seem to be approaching adequate so on comes the FOH. "Is that gonna be that loud for the show?" I say 'You bet'... since the rig was barely breathing at that point. So they immediately have to revisit their whole stage sound because, oops, they can hear the room with the rig on. Waste of time?
So after being asked, I do everything I can to keep the house way down just to get this over, and one of the performers comes out to hear and he actually says, "It sounds like it's all from there (the stage) and isn't out 'here'. Can you do anything about that?" I said, 'Well, that's kinda the opposite of what you were asking for 10 minutes ago.'
Sound check ends, and I know they still weren't really happy with what they had.
Sometimes, when troubleshooting a problem, turning FOH or Mons off to hear what's going on is necessary and smart. Doing an entire monitor mix with FOH off is asking for a waste of time, and big trouble when the house comes on, yet I know many will still keep asking for it...
I will note that while approx an hour was spent on what the band heard, I was maybe given 15 mins with the house on to get it together for the people that pay to hear them. Note that you'll never hear... "Let's get the house mix together first, and then do monitors."