Generally we tip for a good service rendered;  

We tip at the bar or club to ensure hard drinks and quick service

We tip at the restaurant because we want to be comfortable when eating and a good server can make that happen so we take care of them

We tip our shoe shine boy, shines them nicely for a couple bucks mas

We tip our baristas for knowing our order and getting it going before we even get to the register

We tip the car wash guys for that extra shine

Why wouldn't we tip for good service while we are getting our tip serviced?  

I tip my regular gals everytime for a great time.  I'm sure they appreciate it, as I appreciate their service

But depending on who you select and what rate  you agree on, a provider may upsell or ask for a tip because they feel low balled.  To avoid that, pay the asking rate and read their reviews.  As an example, some really good providers may charge more, but there is no tipping, no counting of the money, no upsells and never really ever discussed.

I can't imagine not tipping.
It's like a cab or a restaurant.
If the service is great I tip more but unless the provider is a total rip off I always leave something. It's proper etiquette.

I rarely tip. But I usually see ladies $300 and over. At that price point, you are already paying a premium price so you should expect excellent service. I usually tip lower paid agency girls, $250 and under, but generally not $300+ independents (including TS's). Sometimes I'll drop an extra $20 if the service was particularly good. Ladies aren't bashful about raising their non-negotiable prices when interest picks up, there is no reason for you to pay more than their asking price unless you choose to. And I have seen a lot of ladies and repeated with a number of them and most were more than happy with the asking price.