I'm not sure that an escort who doesn't report her income, let alone her tips is committing anything "immoral".
If I were an escort, I wouldn't report my income to be taxed. Here's why:
Generally, I think, escorts can't stay in the business too long, because after a certain age, they can no longer make much money in the business. Having the government take a large chunk of your hard-earned money is not in the best interest of the escort.
It makes sense for escorts to try to make as much as they can while they can, and use their money to better their lives. It also would be wise of them to prepare themselves for a more stable profession with that money.
Some escorts will try to maximize the amount they make by requesting extra fees for certain services (basically "milking" the client as much as they can). Others will try to maximize their tips by perfoming great service. And others will try to maximize the amount they make by making their customers as happy as possible so that they will repeat. In all cases, the goal of the escort is to make as much money as possible.
Those who try to milk you are only using a tactic that will backfire, since in the long run, they will end up having less business because of less repeats. The exception, of course is if they don't plan to be in the business long, and just want to make as much as they can in a very short time. There will alway be naive clients who, temporarily blinded by their sexual needs, will agree to shell out another 50 bucks to have the escort take her clothes off (it's happened to me before).
Ultimately, I guess whether one tips or not is a matter of personal preference and bank account. I would condemn anyone who didn't tip a waittress who provided great service, but a john who doesn't tip an escort who makes more in an hour then they do in two days is not necessarily committing anything wrong.
Having said that, I still think that good service merits appreciation. And what the escort wants/needs the most is usually money, so a tip would be appropriate.
I was just curious to see how many people here tipped, how much, and whether it was before or after. Judging from the responses, I think 70% don't tip/believe in tipping. And those who do always do so at the end.