I'm not too keen on the free-for-all; I'm not big on singing over other people, and as an unaccompanied singer I can't start strumming by way of an advance warning. (The 'poker chip' idea would get round this, although I'd imagine it would need to be a fairly small group.) Plus a free-for-all session can be blighted by a single performer whose self-confidence greatly exceeds their ability.