The ethics and economics of tips while traveling on a budget deserves explicit treatment because the guidelines differ enormously by country.

Countries where tipping is expected and not optional: United States (15-20% for table service is effectively mandatory), Canada (similar), parts of the Caribbean catering to North American tourists.

Countries where tipping is appreciated but not obligatory: most of Western Europe (10% is generous, rounding up is fine), Australia and New Zealand (not expected, appreciated), Singapore and Hong Kong (some tip, most don't).

Countries where tipping can be offensive or create awkward dynamics: Japan (tipping is considered rude in many service contexts — the service is included in the price as a matter of professional pride), China (similar to Japan in many contexts though changing in international tourist areas).

Countries where tipping is appreciated and carries meaningful economic impact: Southeast Asia, India, Latin America, Africa. A $1-2 tip on a $3 meal represents 30-60% of the bill. Your guide who spent 8 hours with you earns a meaningful percentage of their monthly income from tips.

My guideline: tip based on local context and the economic differential. In a cheap country where I'm paying $2 for skilled service, the $0.50 tip I'd consider adequate by dollar amount is genuinely insulting. A $2 tip on a $2 meal is generous and appropriate. Calibrate to the local standard, not to the absolute dollar amount.