A couple of times I've said to girls charging £150ph that my self imposed maximum is £120ph, a rate to which they have agreed to see me at and service has not suffered, in my opinion of course. Any discussions over rates have been done prior to meeting, once the price has been agreed I wouldn't try haggling after arrival. If you don't ask you don't get. Always make sure if you are asking for a discount, be polite, get the offer out of the way early (so as to not waste anyones time if they aren't willing to drop prices and you aren't willing to pay the higher rate) & not to come across as a dick
In any other service industry, it's perfectly acceptable to ask for a discount, so why not whoring?
I did a fee quote for some work the other day and was contacted by the prospective Client to say that they wanted to go with my quote, but would I knock 20% off. I thought about it and offered to knock off 10%, which they accepted. Some people just like to screw down a price so as to think that they've received the best deal.