A lot of good points have been made in this forum. I would have to say, I have actually run into this situation, at the university level. At auditions, and callbacks, I have had directors, OFTEN, ask me about specific people. Here is the standard that I apply to: is it personal or is it professional. I have had personal problems with actors in the past, some even to the point of attempting to attack me outside the theatre because of something I did in rehearsal(aka, marking them late, telling them to be quiet, even for not defending them in front of the director because they were not learning their lines). However, did this affect their performance and relationship to the cast? no. Therefore, I didn't speak about it, because it didn't affect the show, just me and them. I have a very thick skin, and I like to say that I can get along with anyone, even people I have a problem with, and I don't let that get in the way of someone's career. I have never had to actually talk bad about someone, because if they were a problem, I was usually able to find a solution to manage them, and help the situation work itself out.
In theory, as SM's, we have the power and knowledge in some ways to ruin someone's career if we play it well. However, this is two fold , because it can also destroy ours. Therefore, I try not to say ANYTHING unless specifically asked.