In my work as a guitarist I’ve had guitars that couldn’t stay in tune. In the years when I was gigging 4-5 nights a week, I couldn’t be bothered to problem solve (and there weren’t as many forums to seek advice back then), so I flipped quite a few guitars - basically, if it wasn’t good at staying in tune, it wasn’t giggable to me.
I’ve been in situations where tuning between every song is worthwhile, but that generally means I’ve also got a front person who engages the crowd between each song, or maybe I don’t play at the start of every song, so there are a few seconds to check things before I come in. These generally aren't bar gigs, though, so people are actually interested in the front person and banter improves the show.
In bar or wedding gigs, sometimes you just want to go from song to song without tuning much - at least 3 or 4 songs generally. Most of the time, after 3-4 songs, you need some sort of change of pace, just to keep the audience awake anyway. Plan your sets this way - if the music has to keep going, maybe there is a song that starts with just drums and bass, or keys, or something that changes up the musical texture while also giving your guitarist a chance to check tuning. It not only gives the tuning opportunity, but also will help perk up the ears of your audience instead of having an onslaught of "everything sounds the same all of the time."
Now a few specific tidbits to help all of those guitarists out there (some of these might be good for bass players too):
1) I know it was years before I learned this, but when tuning, you should always tune “up” to a note, not down. If you a string is sharp, lower it more than you need to, then tune up to the note - this helps A LOT with tuning stability. When you tighten a string, it pulls it snug. When you loosen a string, it sometimes doesn't loosen fully because the string catches on something. So go further down and tune up to the note to compensate.
2) Despite being great instruments, if he is playing them, Gibson guitars are notorious for having g string tuning issues. There are lots of things that play into that, though. Getting the nut looked at and properly cut and polished by a good guitar tech can also do wonders, as Gibson’s tuner placement means the strings can catch there. Likewise, adding “nut sauce”, graphite or some sort of lubricant when you change strings helps. If, when tightening/loosening a string, if you ever hear a "ping" sort of sound as the tuners are adjusted, that's a pretty big give away that the strings are catching on something.
3) Making sure the tuner bolts (I don’t know the technical term) on the face of the headstock are tight makes a huge difference with Gibson and Gibson style guitars too. I didn’t realize these things got loose until I had been playing Gibsons for 22 years! I had blamed the tuners in the past and spent money replacing perfectly good tuners on guitars at times that might have just required a little tightening of the bolt on each one. I now check the bolts every time I restring. This was a life changing discovery for me!
4) I recently also got something called a "string butler" for my ES-335. That guitar had tuning issues before, but now is close to my telecaster in terms of tuning stability. But I think the string butler might be more of a "band-aid" for an otherwise incorrectly cut nut. But it is a band aid that works, so...ymmv.
5) When you put new strings on a guitar, pull at them and stretch them out too. This helps tighten the strings at the tuning post. When putting on new strings on an electric, I pull and stretch them until each string actually remains relatively stable.
6) Speaking of new strings - I learned this next tidbit from an engineer. When you put new strings on, stretch them until they stay in tune, then leave the guitar for 24 hours or so. Metal becomes more elastic as it is stretched, but then hardens when under consistent tension. So when you first put your strings on, the string becomes more elastic and is also its weakest. This means your tuning stability isn't as good AND it is easier to break the strings. By putting them on, then waiting 24 hours, the string will settle in to the tension, have better tuning stability AND you will break less strings.
Most of the time, this means having extra guitars around though, so you can still practice while the strings settle in on the other guitar...
7) make sure you use a tuner that is quick and reasonably accurate. Years ago the boss TU-2 became the industry standard not because it was the most accurate tuner, but because it was quick and accurate enough for live work. The TU-3 that replaced it is even better. TC electronic's polytune is awesome (and the poly mode can help you quickly identify which string is out of tune). I know that things like strobe tuners can be super accurate, but it is hard to be fast with them, and speed is important in a live scenario. I wouldn't use a TU-2 to set my intonation, but it sure does a great job live.
Oh - and make sure you have a pedal or clip on tuner. I can't believe there are still people who will unplug their guitar and plug into a tuner to tune up. That's fine for practicing at home, but live you need to be prepared to fix things quickly and on the fly.
8) Tread carefully telling people this last one, but lastly, consider that maybe it isn't the guitar...
The first of that is one's technique. Guitars are never perfectly in tune, and your fretting hand can easily render a note sharp with too much tension. Likewise, picking/plucking too hard is tough on strings and your tuning. Having taught lessons for years, I've had a number of students where I can tune up their guitar, have it play perfectly in tune, I hand it to them, and all of a sudden things sound out of tune. It's not the guitar in that case - it is them. It is easy to fall into bad habits when nervous, excited, or under some sort of influence while in a gigging situation. If you find yourself tuning a lot, you need to ask - is the guitar going out of tune a lot, or is it me? What is the tuner telling you?
It can also be your ears/mind. I've even had situations where due to an ear infection or cold, my hearing just keeps hearing myself as being "out of tune", yet my tuner shows me everything is great. A few years back I went through some rough stuff in life and was having panic attacks, and for some reason, whenever I was close to a panic attack, music would sound out of tune to me. My tuner would show me as being in tune and I would ask my fellow musicians (whose ears I trusted) if they were hearing that, and consistently they heard everything as fine. But boy, was I ever checking my tuning a lot in those situations, because I couldn't trust my ears anymore!
Oh, and lastly, though certainly not applicable to everyone - I will also say I LOVE telecasters for their tuning stability. My custom shop '51 nocaster could literally get tuned once a month without worry of it going out of tune... I still check it every few songs though!
