Well, the stakes are more or less lower with high frequencies, but a crappy guitar tone can also wreak havoc on a mix. And then the best you can do is pull a ton of mids/treble out to muddy it up and de-harshify it. Which won't make it good but might make it bearable.
But as a sound guy, I try to give musicians the benefit of the doubt that they can dial in something usable. If the initial stages of the soundcheck show otherwise, then I'll make some suggestions. After 2 or 3 rounds of that, if they don't follow my advice (guys, you DO understand I'm here to make you sound good, right?), then I say to hell with it and just try to do the best with what I have. So far, old british guitar players are the worst for listening. Like, dude, if I'm telling you to turn down IT'S BECAUSE YOU'RE TOO F'ING LOUD. I don't say that stuff because I hate guitar, I say it because I'm trying to get a good balanced mix out in the room that you can't hear on stage.