Sales tax is just over 8%, so doubling the tax is an easy way to approximate 15% for those who have trouble with math.

Standard for cabs is 15%, not $15.

For restaurants, 15% is really the minimum of the service was acceptable; I think 20% is moving towards becoming the norm.

Personally, I'll only tip the hotel doorman if so extra effort is required (like spending a few minutes trying to attract a cab during a rainy rush hour).

Of course, Flyertalk has 125,000 members and probably at least that many different opinions on the topic.