Tipping practices vary widely by country. In strong tipping cultures (US, much of South America, parts of Southern Europe), restaurant servers and hotel staff often depend on tips for living wage. Tipping generously costs you little; means significantly more to them.
Where to be generous
US restaurants: 20% minimum, 25% for good service. Latin American service workers. Hotel housekeeping staff (often missed). Tour guides who genuinely added to experience. Drivers who went beyond.
Where it doesn't matter as much
Some Asian countries (Japan, Korea) where tipping is unusual or rejected. Most of Europe where service is included by default (10% extra is generous).
Research before trips. Generous tipping in tipping cultures is one of the small ways travel can give back beyond the photo opportunity.