"a few years back deliberately circulated "the number for tonight" within their group, so that the regulars could adjust their tuners to A=394 or 437 or some such, to prevent "walk in" wanderers from (easily) playing along."

How nasty and petty can you get?

Of course, there are occasions when you need to do a bit of "fine tuning" if the lead instrument in a session isn't exactly in concert pitch. Understandable with some reed and wind instruments although there's really no excuse for string and fretted instruments.

There may be a good "musical reason" for wishing to tune an instrument slightly sharp to sound a little brighter but, on the whole, I think it's a good idea to try and play in concert pitch when you can.

I'm quite good at tuning "by ear" although I usually require a reference note to start. After that, it's fairly easy.
However, a clip on tuner is still handy especially in loud sessions(It is less disruptive) but, of course, it has to be at the correct pitch itself.