Here's the situation, and I've been in it myself with salons, masseuses, etc.
A client comes to see me, has a great time so he decides to leave a tip. He's happy, I'm happy, we're all happy. A few weeks later he books again. Now he's wondering, "huh...do I tip her again this time? Same amount? What if I have a better time than the 1st time, do I tip her more? If I tip her less, does she think I don't like her as much, had a worse time than the 1st?"
You see, he has complicated the relationship. He now feels as if he needs to tip me every time to not look like a cheapskate. He has up-sold himself. All other clients are paying me "x" but he feels obligated to pay me "y". Instead of excitement and anticipation for our next date, he is nervous about offending me and unsure of what to bring. I am using the 2nd date as an example, but this can happen on the 3rd, 4th, 10th, 17th, etc.
I set my rates at what I feel comfortable receiving for the given date. I do not expect any more than what is posted. Leaving the requested amount is a win-win situation.
There is one exception: special dates such as holidays, birthdays, etc where you can state "I left you a little extra for Christmas" or "I wanted to give you something for your birthday". That way it is not expected the next time.