I remember reading a saying that amateurs practice to get it right while professionals practice so they can't get it wrong. No-one's being paid at a singaround so it's fine to fluff it as long as you've tried to get it right. As for starting another I would agree with previous contributors that it's OK if you've just started and forgotten the first or second verse but after that it seems a bit self indulgent. All down to context and numbers of participants as well though.