maybe I'm just reading too much into your question, but what if you thought about it in a different way.

If I was an employer, what would I like to know about my own company that I don't currently know?

If you can be helpful on your way out, without being critical, angry, bitter, or vindictive then be all means do so. Indy's experience above is a great example. If you can't say anything without anger, then best not say anything, but if I had an outgoing employee tell me that one of my managers was problematic, you bet I'd be listening. Two sides to every story, but smart leaders listen to problems and fix them. As long as you don't' become a problem on your way out, might be mutually beneficial.