If you see the same girl several times and have built some sort of a connection, they will let go and enjoy the session as well. I see two amp girls regularly and I always make a point that they O during DATY. And you can also tell if they are faking it.
In fact, I had a session last week and no matter what I did I couldn't make her cum. She was exasperated and disappointed that she couldn't cum because she was just too tired. This was almost 1 am. She could have faked it so she could get rid of me but she knows that I know when she is cumming.
Yes, most of these girls are in for the money. But every now and then you find someone who just want to "rub one" out during your session, to me that is a bonus.