No good soundman minds being asked to adjust a mix - but remember that the optimum balance is a matter of preference.

Most soundmen object violently to the two fans of the two different guitarists who both want their mate louder than everyone else.

Personally, I hate people who think "harmony" means their mate out front of the mix and the rest should be bvs.

I also hate people who (typically) ask for "more bass" when they mean "more low mid".

I'd rather have a nice warm vocal sound than lose that feel simply for the sake of "clarity" (which often means a speech contour that makes the voice sound like a mess of sibilants).

I always make a point of finding someone who has heard the band frequently through other PA rigs and knows that band's live sound, to check that his ears tell him the same as my ears tell me.

I also hate sarky conceited gits who say "take the "honk" button off the monitor". Yes I have heard it said (not to me, I was just in the audience but I was ready to get up there and thump the "star" for his total lack of manners). The soundman cannot hear the monitors (I use headphones plus a small hifi amp driving headphones from the speaker outputs (!) to make sure I can hear the monitor mixes over the front of house, for brief moments as long exposure to that level would send me totally deaf, but even then I cannot tell how loud the stage monitors are) and he depends on the band to make it known what they want in the monitors.

If the band has on-stage guitar amps, then they are usually turned up by the band until it is essential to drive the vocals as hard as you can to get the vocals over the stage amps. One venue with a grunge/metal band I was using the house rig with my mixer and the band put the guitar amps so high the 14 kilowatt house rig could not make the vocals audible! This was in a venue with a capacity of about 300.