I think that most prostitutes regard french kissing as the most intimate thing on the menu. I think that's evidenced by the fact that it seems to be the only thing that they will withhold due to their "discretion". They're sure that they'll suck anyone's cock but a snog, well, let her have a look at him first.
So, if they withhold it, or scrunch up their lips so's not "to allow easy access then I'm already feeling bad about the whole encounter. To me it's an essential.