A question about protocol and politeness.

Among regulars at a singaround, what do you think are the appropriate circumstances for telling another singer, in a helpful spirit, that his or her singing was flat/wandered off key/gave out on the high notes/etc?

I think my own answer would be that you shouldn't do this at all, unless the singer has started the conversation by being critical of his or her own performance - and even then you should tread carefully unless you know the person very well. I guess I don't think technical perfection at a singaround is important enough to outweigh the nasty surprises & hurt feelings that this could involve.

But is that over-protective? Might some singers benefit from being told they'd been screwing up without realising it? Has unsolicited criticism got a part to play in maintaining singaround standards?