The question of how much cash to carry vs how much to rely on cards changes depending on the destination, and the answer isn't always obvious.

Cash-heavy destinations: Vietnam, Cambodia, Myanmar (when accessible), much of rural Africa, smaller towns in Central America. In these destinations, cash is king, card machines are rare or unreliable, and having sufficient local currency is a genuine practical necessity. Budget extra cash buffer — being short of cash in rural Vietnam is more stressful than an overdrawn account at home.

Card-heavy destinations: Western Europe (contactless payments are ubiquitous, many places are cash-free), East Asia (Japan is famously cash-heavy but QR code payment is universal; Korea and China have excellent digital payment infrastructure), Australia and New Zealand.

The nuance within countries: major cities in even cash-heavy countries increasingly accept cards at tourist-facing businesses. The street food stall in Hanoi doesn't take cards. The airport restaurant in Hanoi does. Plan accordingly — arrive in a country with enough local cash for the first 48 hours before you need to find a fee-free ATM.

The currency exchange strategy: use ATMs for local currency rather than currency exchange booths whenever possible. The exchange rate at a bank ATM is the mid-market rate; at a tourist-area exchange booth it might be 5-10% worse. The exception: countries where ATMs charge high fees or where the black market rate is significantly different from the official rate (Argentina, historically).