I tip at places where I see people fixing my particular order. I tip at other places where the service is good. I tip the Pizza Delivery guy because that is most of his income.
I tip an average of 15 percent in restaurants. It is unfortunate that restaurants will not pay decent salaries and eliminate the need for tips. Most restaurants and other places collect the tips and then divide them among the employees so that the amount that has to be reported to the IRS is reported and the employer pays his share of the Social Security Tax, which is not always require, but sometimes.
I have hosted a few large banquets at hotels. Some places add the tips before figuring the taxes, thus you are paying taxes on the tips. Others levy the taxes and then add the tip. If you put on a $20,000 event and there is a mandatory 19 percent gratuity, the point at which the taxes are levied can make a difference.