Most trips should not rely on a single payment method. A resilient setup uses a primary card, a modest amount of destination cash and a backup method, with overseas transaction, ATM and dynamic-currency-conversion fees checked before departure.
When cash is useful
Transport, small merchants, local markets, deposits or temporary terminal failures can require cash. The amount should reflect local payment habits and trip length rather than an attempt to perfectly time a short-term exchange rate.
Choosing a currency at checkout
If a terminal offers the local currency and the cardholder currency, inspect the rate and extra charges first. Dynamic currency conversion moves the conversion to the merchant or ATM provider and can have a different cost structure from card-issuer conversion.
Building a travel budget
Use a dated reference rate for accommodation, transport, meals and activities, then add card fees, ATM fees and a reasonable fluctuation buffer. Actual spending should be reconciled to statements and receipts.