"Little trick from an oldtimer. Tune just a touch sharper than the other people you're playing with. Makes you sound more lively. "

I noticed the other day that a harmonica I was buying had been specifically made to do just that for that very reason.

Of course if everyone does that and kept on doing it, it wouldn't be that many years before the only people who could really hear the tunes would be bats and maybe dogs...

And there are plenty of musical instruments played in sessions ove er here anyway that are not at all readily tunable, and in many cases impossible to tune for practical purposes (well you could send a squeeze box away to have its reeds retuned, but sessions aren't normally that extended...). And even with the ones that can be tuned - well, can you imagine a guitarist with the nerve and discourtesy to ask a hammer dulcimer player to retune the whole set of strings rather than retune the guitar?