Manners are in the ear of the beholder, and quite subjective.
During the routines of regularly scheduled open mics, showcases, and other events, there are many opportunities to demonstrate good character and practice courtesy. The effects are not only professional, they are enriching.
It's good to let the audience know a little about yourself, including accomplishments. As a courtesy when I emcee a showcase set, I sometimes make it a point to conduct a little interview so that the audience may get to know the performer. This enables me to convey to audience some of the songwriter's accomplishments, such as songwriting awards, sparing performers from boasting! Thus, they don't have to "toot their own horn." Whether or not awards are involved, I can find something interesting to discuss onstage during my brief conversation with both the performer and the audience.
During subsequent songs, I listen with more warm familiarity and respect for the performers.
Sophisticated performers can take care of this without an interviewer.
Here are things I personally like to hear (or otherwsie sense) at shows:
"Let me get my guitar case out of the way." Sometimes musicians get swept up and leave their cases on top of a table where patrons should be allowed to sit, or they otherwise inconsiderately create a dangerous obstacle course.
"I hope these songs leave you with a good feeling." Songwriters sometimes forget that their music can strongly affect listeners. That's a goal, and the effects should be good. A saying is (paraphrased), "It's the job of the folk singer to comfort the disturbed and to disturb the comfortable," but that's only up to a point, perhaps to describe a need for social reform. Audiences don't come to show to be made miserable. Levity helps, as opposed to being too serious throughout the show. Make some eye contact and smile--build empathy.
"Please tip the waitstaff, whose names are ______."
When I learn who's serving, I write down their names. It just takes a minute. A friend told me that his courteous acknowledgment of the waiters and waitresses at The Bluebird in Nashville netted him coveted weekend shows there as importantly as the songs he played. The staff gets to vote.
"That song was written by ____." Sometimes I've seen musicians cover a song in such a way that they imply that they wrote it. Of course that is unethical. Conversely, some shy musicians are afraid to admit what they wrote!
I love to hear "I practiced these songs," as opposed to, "Gee, I wonder what I should play?" which makes me uneasy and feel disrespected. I'd like to think we won't end up hearing some random, unrehearsed songs that will not be done competently.
"Can you hear all of the lyrics?" Articulation and mike technique are important, because every syllable must be heard. Don't mumble or mutter, and find the right distance from the microphone. Back off for high, loud notes!
"Let me know if my guitar is loud." The best guitarists start with their volume off and work their way up to a pleasant, audible level, and there they stop. When I hear a guitar turned on at a high decibel level, I expect the blend to be nonexistent.
Vary the tempos and rhythms of your songs. As a drummer, I'm sensitive to rhythms and tempos. "Those first two songs were fast ones. Here's a slow one for a little variety" is music to my ears.
"I made sure the next song is only three or four minutes long so you don't get weary of it." Too much length can turn a good song into a disaster, past the point of diminishing returns Violators who extend a song that "goes nowhere" are said to "beat a dead cow (or horse)." It's not a good feeling when I'm listening and I think,"This song should end right about HERE," but then, it keeps grinding along. It's a bad moment comes when I realize, "Oh no, here we go again" with another dreaded verse or chorus after ear fatigue has set in. Time the songs--you want the ride to be a good trip, not an endurance marathon with listeners looking for the exit.
"I'll stop a few minutes early so the next performer can have a little extra time." People who say this should be nominated for sainthood.
"Sure you may borrow my guitar," or pick or tuner or capo, etc.
"I've taught my friend here to accompany me, so the show will be more interesting." As an accompanist, what can I say?
"Thanks for listening; you've been a lovely audience."