I think the only times it's acceptable is when you find a true confidante and if that person has complaints too, then the two of you can commiserate together. I had that situation in one job - actually, it was with two others on my larger team and the three of us would eat lunch together and would complain about our boss. It was cathartic. So, I think it's OK under certain circumstances only. But I agree with you that if it gets to the wrong person, it could be a bad situation. And if it's just one person complaining openly to everyone, then that creates a negative situation.