A lot of interesting chat and opinions on this thread but it all boils down to respecting each others efforts and not imposing ones own versions on someone elses singing/playing.
If someone is performing; paid or amateur, singer or musician; then for that time they are the lead and their version is what goes. They might have developed a variant tune or pace or whatever that actually enhances their own efforts and even if you hate it you should show it some respect and not try to impose a different version.
Over the years I have been to many concerts/festivals and folk clubs and come across many local rules which I respect. I have also been helped as well as hindered by people joining in when they thought I was in need of help.
This has lead to a piece which I perform from time to time.

FOLK CLUB ETIQUETTE

Here's a little guidance about what you shouldn't do.
For regulars at the folk clubs as well as those who're new.
It's really very simple so please try not to forget;
As I teach you all the basics of folk club etiquette.

We all understand the powerful pull of the bar,
That overpowering urge to buy another jar.
But please remember, use a little tact.
It's rude to leave the room in the middle of an act.

Most people fail to realise folk songs have many versions.
Some of which YOU might think are better called perversions!
But always remember that the artist's never wrong..
And you must use their version if you're going to sing along.

Sometimes you may decide YOU want to do a spot.
No one will object as long as you give it your best shot.
It really doesn't matter if you sing or play or recite.
But don't forget to name the writer. Not to isn't quite right.

One thing is certain to get on any singer's goat.
Guaranteed to rankle and cause a discordant note.
That's the racket from the packet of your favourite little snack.
If you're a noisy muncher, you won't be invited back.

But by far the greatest Folk Club Crime,
Is heedlessly wasting another man's time.
By slipping in a second when you're asked to do one song;
Or stretching an introduction far, far too long.


And if you don't like the tune or song.
And you fancy a pleasant little chat.
Well I'm here to tell you it's wrong.
So please don't even think of doing that.

If you are enjoying hearing folk sing
Please don't let your mobile phone ring.
'Cos if you're the cause of that disruptive sound
you'll leave the club without touching the ground!

And if you don't like the words or the music.
You must not indulge in a manner anarchic.
Just sit there quietly until there is a pause.
Then you can leave put please don't slam the doors.

So please remember - Don't forget -
The rudiments of FOLK CLUB ETIQUETTE

I don't do all the verses all the time but I'm sure you get the idea.
Jim