If a lady is out there posting escort ads, I believe it's implied that reviews can be posted as well.  If a lady has herself on the internet (can't get more public than that!), then I have the right to write a review.  That said, I have seen several providers that had a "no reviews" policy and I've always respected that.  Each of those ladies said that they were very low quantity providers and that they wanted to maintain a low profile.  If that's their policy, then I won't violate it.  Otherwise, it's fair game.  And I'd agree with NYE, if the provider is fake, ripping people off or something similar, then all bets are off and a review is definitely appropriate.

It shouldn't be a problem if she already has a bunch of reviews already, unless it specifically says something different on her website, it should be fine to just write a review, doesn't matter if its an agency or an indy.

As has been said. If you're happy with her, as others said, you might ask first if she's not reviewed already. (If you're a newbie, don't be seeing non-reviewed women anyway.)

To go a step further, never mention extra time or OTC time. If she does something with you that's not on her profile, or more importantly her own site on her "menu", you might pause and consider whether you should include that. A lot of women are VERY YMMV, and rather than have to turn down lots of guys asking for a given service, they'd rather we all walk through the door thinking she doesn't and then she will do it when she's comfortable. CIM is a good example.

Have fun!

asking the lady first?  
Some ladies wish not to have reviews. Same as references, some are references friendly and some are not