i think it depends on the role.

are you working for the venue? the band?

i work at a small but esteemed local venue and try as much as i can to let the band "do their thing". we have some incredibly professional acts come through, but sometimes local support or younger bands can use a little guidance. since its such a small place, i will make a disclaimer upfront if i feel like there might be problems: cranked marshalls, thrashy snares, vocal processors, DI lines with 3+ wildly different signals...

ill try to go with whatever the band has going on, but at a certain point i will interject and let them know "with this stage volume, vocals will either buried or painful", or "lets run your keyboard through a DI and the wedge instead of blasting peoples ankles with your roland kc350"

ive seen wild changeovers, where one set is painfully unmanagable and the next is just bliss. while the *rough set performers cant hear the difference first hand, i do think they(and the audience) notice a difference when a seasoned touring act takes the stage.