I think private show ratings are pretty good at the moment. I do sometimes have problems when a user will ask for something I don't offer in private and they get very disappointed and end the private show as soon as I set my boundaries; which is probably why I get a few unsatisfied scores. Its very obvious what the model offers when you click the private show button because you can see a full list of things they would do in the show. I usually ask that the user PMs me beforehand to go over what they are interested in for private, but not every user respects those wishes and will start a private with no information on what they'd like to see.
I believe that many users want privacy so the rating being anonymous is good for them, but I get users that ask for something like full or hardcore anal; which I do not offer in any of my shows. I tell them "I'm sorry I'm not comfortable doing that and don't offer it in my shows" and they end the show right then. I believe those users most likely give me an unsatisfied rating. Same for users that ask for something that's against the rules (but most of the time if they ask for something against the rules I warn them or report & ban them) I'm not sure of a solution for this but, I hope the feedback is helpful!
If you also are thinking about making a public show rating, I would only add that option for people who have tipped a certain amount.