If you havent been given a sound check you can't be expected to get it right on stage, you've got to look after FOH.
I put only vocals in the vox mons unless its a massive stage.
I bought some amp stands for keys and guitars, and point the cabs at the players heads, that generaly keeps their level below 10 and it doesn't need to be as loud through FOH so more headroom for vox.
The larger/est sound companies have FOH in mono, that way all the audience will hear similar levels from across the stage.
The above is before 9am so take it with a pinch of salt...