I absolutely avoid trashtalking, and go further-- if a partner tells me something about a conflict they have with an OSO, I am very careful to communicate (1) that I can provide a sympathetic ear, to a point (I have careful boundaries around not getting too involved with their conflicts), but (2) I am not going to voice my complete opinion about the subject, because if I do think anything negative about their OSO, I'm not going to say it.
I am generally all about complete and open honesty, but this is one exception. If my boyfriend's girlfriend is in any way in the wrong, I will go out of my way to avoid voicing my opinion about that, if at all possible. If that becomes a problem, then there is a deeper relationship problem that I need to address.
I have never had a problem with a metamour saying something negative about me, but I would set some serious boundaries if it did happen (which might limit or end my relationship with that partner).