On analog boards (pre fader, post EQ), I try always to run separate FOH and MON channels for the main artist /lead vocalist so FOH changes will not affect the monitor mix. Don't usually have enough spare channels to do the other vocals. When running one channel for both FOH and mons, I am surprised at how many people can tell when I am making subtle changes in their EQ for FOH, since it affects their monitor mixes. Some notice everything, and others don't seem to notice or care, so long as there are no feedback issues.
On digital boards, I set up as many duplicate vocal channels as I have available so they all get separate monitor channels. I try to spend time getting the stage monitors to sound great with the vocal mics. I think that goes a long way to having happy artists. Once you get them done, you don't usually need to adjust again except for overall volume as needed. The ability to use an iPad or tablet to be on stage and stand next to the artists while working on their monitors is invaluable.