The thread title is about folk clubs. These are usually concert-type events where the performer is paid a fee and the audience pays to get in. In some cases, rather than pay a fixed price on the door the hat will be passed around, but it is usually understood that this is a donation for the music and not a tip. Of course, if anyone wanted in addition to tip the musician it would be rude to refuse, but should not be expected.

Where a group of musicians are playing in a pub session and someone offers to buy drinks or give a tip, then it would be ungracious to refuse. In some circumstances, for example where the session is in effect an informal concert intended to entertain the pub audience, then it might be acceptable to pass the hat. However most of the sessions I play in are primarily for the enjoyment of the musicians and it would not be acceptable to put the hat round any audience who may be listening.

The only situation where a tip (and a tip jar) might be acceptable is where the musician is providing background music eg in a restaurant, where they are effectively in the same position as the other staff.