h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

In New York City, the general rule is to always tip when there's any question at all. On the rare occasion when someone is not to be tipped, this will be made abundantly clear to you beforehand.

I suppose this culture arose because of the high immigrant population. Thanks to the numbered grid structure of Manhattan's streets in particular, somebody who can't speak English at all can easily deliver food, flowers, or packages. Such people are often working for distant relatives, friends, or somebody their friends or relatives know well. Chances are the mom & pop shop they're working for can't pay much.

New Yorkers prefer to give new arrivers a leg up. Many natives are descendents of immigrant parents or grandparents themselves. So everyone is aware that this is how it works. The newcomers work in the back of the shop at first, graduate to deliveries, then maybe working at the front counter where they speak English all the time. Eventually some will apply for citizenship, and from there the sky's the limit.

So the rule is to tip and to show proper appreciation for correct English coming from someone who is clearly still learning.

On a side note, I've found that it also pays in New York to overtip the bartenders, especially if you go often to the same place(s). You will often get free drinks at the end of the evening as thanks.