IMO If the band, house, or band hasn't asked you to do anything with the PA, don't. It's not your place. Sometimes, especially in small venues, crappy sound on stage eliminates any chance of the guy running the PA makeing them sound good. Sometimes it is the guy's fault running the PA. In that case, unless he owns the club, he won't be there long.
I spent last week in Dallas doing a convention. The band was on somewhat crammed riser on the back of the stage. Being so close to my amp and playing a guitar with single coils I was getting a hum when playing dirty. Keep in mind this is a production with a budget probably close to $500,000. The main sound guy freaks out over the hum. I explain that that's normal being so close to my amp when playing with distortion. He say's he's never run into before. I calmly suggest he straps a gate on the channel. He says he doesn't have 3 "fucking" noise gates (3 splits; monitors, FOH, recording). I bet the situation would have been much simpler and solved more quickly had I just moved the amp under the riser and asked for it in my monitor. I should have solved my problem rather than asking him to do it for me.