You need to look at this from both sides. If you are not her regular, it's best to give her in advance and get the money out of the way.
Lots of these girls have seen quite a few gas and dash guys in this business, so this kind of shit happened quite often, You are more likely to get a much better service if the girl has peace of mind not worrying about not getting paid after the session.
In my earlier days when I go to SRM and seeing new girls, I always leave the money standard rate plus extra $20-30 tips on the shelf clearly visible to them. You will be surprised how well they serve you and how they relay the message to other girls. At one time I left $200 cash on the shelf. Monica came in, saw the money and asked "is all that money for her?" I said yes. She jumped on me, kissed me and treated me like a king with no restrictions at all.
Try that if you see a girl you really like. You will be surprised how well they treat you. That's what happened with my new hidden gem I found recently.