Yeah... I think it's probably best to just let them figure it out - like you said, Zenme, they're grown ups. In any case, I think it'd be very rare that the next girl would believe the X. It's one thing if they come asking, quite another to have the info relayed to them unsolicited.
Info from my experience, AXH's (now ex) GF was at our divorce hearing, and heard AXH get caught in an entire mass of lies, a few of which were to her. (The biggest one being that he was already divorced.) She heard him admit to some really, really nasty behavior.
She still stated that he was an honest and good man. And, she still didn't believe he would continue to drink or behave in an abusive manner with her, and told me as much. That particular rant of hers was almost like she was paraphrasing from the "You think that you are so special, that it won't happen to you. He'll be different with you" threads in the stickies. Her parting shot after filing for a restraining order against AXH was to send an e-mail to me detailing all of his issues as if it would be a breaking newsflash for me (rather than yet another chapter in an old, tired tale).