You must tip (though it is not mandatory) if you are getting service. Not giving tips or tips less than 15-20% are considered very rude here. I always give tips for 20% or sometime even more if I really like service and foods. Sometime I write a write a thank you note on bill to cook if food is awesome and tell waitress to hand over that note to cook.

Do you know how much is the waitress's hourly rate? somewhere between $2-3/hr. They depend on tips only. Some pizza delivery boys uses their own car to deliver pizza.

Don't you think you should give something to the people who travels miles, any time of day and deliver your food to your door? C'mon people, giving someone 20% of your order doesn't make you poor.

However in Indian restaurant, the restaurant manager or owners don't distribute tips amount among waitress. That is why I hate those owners. In those cases I usually tip cash instead of putting tips on check.