Tipping is one of the most common topics on this board. You can do a search (set the date range a lot further back than the default 30 days) and you will find hundreds of posts on tipping. If you do that and read some of those prior threads, you will find out there is no standard. Some guys always tip. Some guys never tip. Some tip only agency girls. Some only tip for soecial services. Some don't tip but bring gifts. Some tip and bring gifts. Most providers will say "Tipping is never expected but always appreciated." Another common phrase is that the best tip is your repeat business.
There is no standard on whether to tip or not, and there is no standard amount if you do decide to tip. Do what you feel like doing. It is your money. However, think about this: these ladies are not waitresses making less than minimum wage and depending on tips to survive. They make in an hour what many of us take a day or more to make
What is considered a good tip to providers?
...Tipping is not required. It isn't expected either. Some mongers tip some don't.
Some say they tip agency providers because of the cut the agency gets.
I recommend not expecting to tip but if you feel the provider was exceptional in some way and deserves one go ahead.
It is wrong however to feel like you have to tip.
I gotten a laptop, various toys and expensive lubes as "tips" before. Oh and one early morning appointment, I was brought muffin, lol, it's just the thoughtfulness of the gesture is appreciated.
Monetary tips have ranged from 20- 200% of donation rate which has always been awesome to receive but their is no rhyme or reason to it.
Some ladies (myself included) even have a "gifts suggestion" or "wish list" section/page of their website so you'll know what she likes.
When I worked for agencies, I definitely received tips more frequently than I do as an independent. However, I now have more friends who I see on a regular basis, and they do occasionally bring me gifts.
And I love the gifts! 
Tipping is one of the most common topics on this board. You can do a search (set the date range a lot further back than the default 30 days) and you will find hundreds of posts on tipping. If you do that and read some of those prior threads, you will find out there is no standard. Some guys always tip. Some guys never tip. Some tip only agency girls. Some only tip for soecial services. Some don't tip but bring gifts. Some tip and bring gifts. Most providers will say "Tipping is never expected but always appreciated." Another common phrase is that the best tip is your repeat business.
There is no standard on whether to tip or not, and there is no standard amount if you do decide to tip. Do what you feel like doing. It is your money. However, think about this: these ladies are not waitresses making less than minimum wage and depending on tips to survive. They make in an hour what many of us take a day or more to make
tipping is highly welcome!
... i've been given gift cards as well for a tip.