Great Question Ms. Bobbi Rant.
This is a loaded question, and much of it would depend on the circumstances, and what kind of trash is being talked:
1). Was the ex a perv who tried to diddle the kids?
2). Did he/she clean out your savings, steal your credit?
3). Is the person violent or truly unhinged?
4). Venting to BGF or family?
The messenger is always going to be despised no matter who, or why. (Friends and family will support your venting, because you support theirs :0)) The messenger will be blamed for many problems, because those who go ahead into a relationship with a Bad Egg, often knew that Egg while still with Messenger.
They think they are something special, and are still at the point of believing all of the same lies; you the Messenger fell for, when you Loved said Egg.
In the case where children are in true danger, I think it doesn't matter how it reflects on the Messenger, the kids will thank you later. Same with crooks.
Although, as you know many people will prefer to hide their head in Farmville and pretend all is well. Just saying...