Having read nearly all of this thread I thought " no-one's mentioned temperament yet". But I was pipped at the post by Stanron.

This is something that interests me as a fiddler, because I know that in some keys "properly" tuned strings sound way out of tune (which is why "classical" fiddlers use 4th finger instead - and which may account for the "folky" sound of open strings ). Fiddles are normally tuned in perfect fifths, like melodeons, so are intrinsically out of tune with guitars, keyboards of all sorts, and tuners that only show equal temperament; maybe one of the reasons that early musicians didn't like different sorts of instrument playing together (and orchestral string players find accompanying parts to piano concertos unpleasant).

I think that what we normally do if accompanied is to play in "just" temperament, but pitched from an equally tempered bass note, and I suspect singers do much the same.
What do you think? And does it matter after several pints?

Interesting that tuners are mentioned. Surely (except in amplified bands) they are only necessary for people who can't hear if they are in tune or not (and possibly players new to the instrument). And they don't provide the experience of "being in tune" because they (mostly) don't play a note which must be matched.

As many have said here, careful listening along with practice is essential. In the olden days (as my kids say) a tutor would hear a student tune his instrument (probably to a piano), take it from him, put it out of tune, then return it to be tuned again, several times, during the earliest lessons, because hearing "in tune" is basic to musicianship, but requires training.