I had this happen to me recently in Singapore with an overly cute (read that as thought a lot of herself) girl I took out from Orchard Tower. Result was predictable - poorer service and a request for extra cab fare after I had already paid above the going rate. Last time for that-next time I walk.
That said, the problem is that when you get into this kind of back and forth negotiation at what i consider to be the wrong time, it takes away from the mood (for me anyway). So it becomes lose lose proposition.
Moral of the story, settle the terms upfront.