CET: the very best I've ever been at was the Mystic Seaport sing around where there were no turns, and you had to damn well step up (metaphorically speaking if you were sitting down) and sing your song. Everybody who was ready was able to sing

Pip Radish: Sounds like my idea of hell. As them as knows me know, I'm a modest and unassertive type until I actually start singing. (I don't think this is particularly unusual

The "jump in" approach can work quite well in sessions where everyone is able to join in when someone starts a tune so everyone is still able to contribute.

However, in a singaround I think that Pip Radish has an important point. The less assertive may well include some of the best singers in the room. Also it will almost certainly exclude newcomers who will be unsure of themselves and need encouraging to have a go. So round the room in some form, moderated by an MC is best for a singaround. I have been to a singaround which went round the room even though there was no obvious MC. When you had finished, you simply passed on to your neighbour. It was only when I got up to go, that the MC (more like a session leader really) introduced himself and thanked me for coming. That seemed to work very well. I suspect because the regulars knew the form and respected it, so no hogging by the "favoured few".

Even in a session it is a good idea to have a session leader who will keep an eye on things and, from time to time, invite one of those who have not yet started a tune to start one so everyone has an opportunity to start a tune if they wish. I must admit that if it is a "one off" session, such as at a festival, I am quite happy to sit back and let others take the lead.