The things people are saying are not universal. Only some US states have lower minimum wages for tipped employees, and in all states, the employer is legally required to pay them the regular minimum wage if their “tips plus tipped minimum” are lower. In practice, this does not always happen, just like how some people under-report their cash tips to avoid paying tax on them.
Tips should reflect the quality of service and response – if your food is bad or cold, it may not be your server’s fault and you should approach management immediately. (Some states forbid the sharing of tips between severs and kitchen staff, though where it isn’t forbidden, bussers and kitchen staff and host staff may share tips with the servers, but this won’t be known to the customer.)
People should always speak up when their food or service is atrocious. Nothing has less credibility than saying “that was the worst meal I ever ate” while sitting in front of an empty plate. If it was so bad, why did you eat it?
Be wary of restaurants where the menu reads “Automatic service charge of XX% for parties of Z or more.” In some states, this is a service charge which goes directly into the profits of the restaurant – it is not a tip or a bonus paid to the servers, and the restaurant is under no obligation to do so. Be sure to clarify this when one is with a large party.
I have personally eaten hundreds of restaurant meals as a business traveler. I have left a tip of zero only three times – both times because of horrible service and attitude bordering on hostile and deceptive. Once was a server who lied about the contents of the meal (my guest had a genuine allergy and the server said “oh I didn’t take that seriously – everyone says they’re allergic to things they don’t like), another would disappear and come back reeking of smoke, and the third served a sandwich with meat still wrapped in waxed paper, then attempted to deny it. That’s a pretty good record.
Tipping is optional, but it is understood to be part of the culture. I personally don’t agree with bumper stickers which say “Tip 18% or stay home”, but if you are so poor, then avoid full table service restaurants. It is not customary to tip for “counter-service” – fast food restaurants, and a tip of 10% is generally considered generous at a limitless buffet, where the servers/bussers only clear away used plates and refill water glasses.