but I'm a pretty vanilla kinda guy.
They get pretty good at reading body language.  If they see that you are heading someplace they don't want to go they will let you know, but you do need to give them some warning.  If in doubt, ask.

Like - "Where do your want to plant that load today baby?"
Really, by the time you get to that point, and both of you have performed well... makes not one iota of difference.

give some clear warning as the end approaches, then is she wants to continue, its her choice.

In my limited experience.. she almost forgot to tell me to warn her, maybe they take it for granted and forget to say sometimes - she only remembered because I asked a related question (like why did you stop to spit in the middle? ans. to not swallow saliva - well I was a newbie). I also read in a review of someone that they forgot to tell the provider and she like ran to the bathroom and washed out her mouth with mouthwash and seemed a little annoyed.