First the disclaimer...
This is just my opinion (as always). My mental meanderings do not reflect the opinions of TER, other mongers, upscale gentlemen, downscale gentlemen, or any other human being on the planet. There is no right or wrong answer. There is no industry standard. It's all about how you want to do it.
So flame away if you wish (I know this is a very sensitive topic), but understand what I'm about to say has as much weight as "I really like a good cheeseburger." Which I do by the way.

But folks shouldn't get offended if they don't agree with my opinion. It's just an opinion.
My stance on tipping comes in several parts which (again - for me) all point to, and support, the same answer - that it is appropriate to tip. Under certain conditions...
My first reason is that I think about the hobby as a service industry. We gents come in and get serviced. *rimshot* And on that front, I feel it is appropriate to tip for exceptional service. "Exceptional" being the key word. The ladies I choose to see on a regular basis all provide exceptional service. That's why I see them.
The second reason is more philosophical. Call it a kind of sexual karma. I feel that when you are generous in life, in the bedroom, or with your wallet - the universe (and usually your partner, too) will give you that back with interest. I don't have unlimited means (I'm sure none of us gents do), but the hobby is all about disposable income. And I think a little extra in the envelope is money well spent. An "investment" - if you will, in future sexytimes. And building a relationship.
The third reason is - let's be honest guys, the girls are in it to make a buck. Yes, some (possibly many) really enjoy what they do. And god bless their hearts for it. But when it comes to money - a little extra is always appreciated, isn't it? And it's the least I can do to show her my appreciation. And yes, of course, repeat business is always the best form of appreciation.
The fourth reason is for her appreciation. I always want to be at the top of my girl's "like to see list". And if it ever comes down to spending time with me, or another gent, or her friends, or whatever - I want my "appreciative nature" to be recognized and factor into that equation. Maybe that's part of my competitive personality. I want her to be quick to answer my email or text with "Yes, daddy I'm all yours!". No sweeter words have ever been typed. LOL. My schedule is hectic enough - I like my girls to get back to me quickly regarding setting up a date. It makes my life much less stressful. So let's call that little tip an investment in "VIP treatment".
The fifth reason is kind of a catch all. I ask many things that could be considered unique, different, above and beyond the call of duty, a pain in the ass lol, whatever - and I understand to some (ok many) girls that is a lot. For example, my role plays take some time and thought. I expect a lady to read them and understand the character and scene. I also have other kinds of "homework" I may ask of my favorites, too. All of these homework assignments take time. And as we all know, time is money. I also may be a few minutes over sometimes as lord knows I like to chat. So I want to be respectful. And if my girls aren't clock watchers (they aren't) I don't want to take advantage of their hospitality.
So, in my mind at least - all these reasons point to the same conclusion. It is appropriate, respectful, and gentlemanly to tip (for a favorite that provides exceptional service).
The only remaining questions are how, and how much?
The how much - I won't discuss my personal policy, and I would say do what you feel is right. If you want to tip at all, of course.
The how - I have always found it awkward to add it on at the end. Handing her cash seems declasse and weird. As does leaving it on the counter. So, I just include it in the donation envelope up front. Yes, that means I do sometimes tip for exceptional service and then don't get exceptional service. Well, I chalk that up to experience and move on. Only exceptional ladies become regulars of mine.
And I make no distinction between indies and agency girls. They all work hard and deserve recognition and appreciation for their efforts.
Anyway - again this is not in any way an argument for men to tip. I am only sharing my personal opinion on the matter.
And that's my tip to you.
Keep the change.