Good comments by hay man. It is too easy to let your guard down the minute before she answers the door instead of when you have decided everything is to your liking. Whatever you do, don't leave it too late. If she looks horrible, or her attitude is horrible, move quick. If you walk in and the place is too much of a shit hole for your expectations, move quick. Be calm and polite but get out of there. If you feel certain she's a decent sort but a bit of a minger, then possibly a quick reason (if photo inaccurate, looking for someone younger, blonde, brunette, whatever) but never said as if opening a conversation, merely as a polite noise with a smile as you fuck off out of there. What would you do on a blind date you didn't like? Maybe, (1)"I'm sorry, but I don't think this is going to work"; (2) smile); (3) go. All three should all take no more than four seconds from opening your mouth to departing quickly.
But don't get so neurotic that you jump into quit-mode too easily. They're prostitutes, not a hospitality service. If she's worth the money you're about to spend, end of. Paris Hilton probably charges much more.