Tipping is not optional.

Fifteen percent is standard. Twenty percent is for noteworthy service, ten percent or lower is only acceptable in extreme cases of neglect, rudeness, or spit in the food. If you do tip less than ten percent, don't ever go back to that restaurant again - if there wasn't spit in your food the first time, there definitely will be the second time.

Yes, tip your delivery boy. Yes, tip your taxi driver. Yes, tip your baggage handler. In fact, any time you interact with another human being who has to endure the unending shit maelstrom that is customer service for the general public, at least consider leaving a tip.

If you can't do the math in your head, round up.

And if you ever, ever leave one of those gospel tracts that looks like real money until you pick it up, you damn well better leave real money underneath it, motherfucker.