Ditto.
However, and in the spirit of the thread, I think that there are a few instances where a tip may be appropriate:
"Extra milleage", but not necessarily in terms of the alphabet soup only: If you're SP goes the extra mile to please you and make your fantasies come through, tipping is a good way to show your appreciation and compensate her for her extra work and time. For instance, if you are asking your SP to be done up in a very specific way that requires extra care and time to get ready for you (keeping in mind that your average civilian takes about one hour to get ready, do the maths to figure out how long it takes your fav SP to get all done up for you), or to put some extra time into preparing a "scene" for you, tipping is a good way of showing that you appreciate her special efforts.
"Extra service": If you over-stayed your time significantly (more than 10 ish minutes), it's a nice gesture to offer her a compensation for the extra time you stayed with her, especially if you intend on seeing her again. Unless of course she explicitly told you it was OK for you to stay a bit longer "on the house".