A tip on tipping

This is not a discourse on whether or not tipping is necessary. It's a given, to me, that you must leave a tip in the United States when receiving table service.
Some categories of people tend to tip more and some less. Probably the best tippers are current or ex-restaurant workers. I can't say that women tip less than men although they may drink less so the overall bill is lower, leading to less money in the tip. But the percentage is not necessarily lower than men.
Europeans tip less because that's the custom in their countries so if I detect a European visitor I add 18% to the check before it goes to the table and have yet to get a complaint.
My definition of a cheap tipper is someone who tips exactly 15% of the bill before tax. That's a penny pinching miser, and includes a prominent local politician. Not the best way to get my vote.
What is the difference to the customer between tipping 15% or 20%? In most cases a few bucks. That is not going to greatly impact the customer but will make a huge difference to the server. Not just to his pocket but also to his attitude and mood.
Remember, if something doesn't go right with your restaurant visit, it very often is beyond the server's control. Don't punish him because the sous chef called in sick or the dishwasher went on a bender. If I eat out and a server is truly negligent I tip him 15% and either never patronize that restaurant again or ask for someone else's station when I go back.