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I have been in Thailand and Cambodia three times in each country. Each time I used my Schwab card 3-4 times.

Yes, Thailand withdrawal fees for overseas card withdrawals in ATMs are about $5-$7 but in every case these fees were refunded by Schwab automatically, without even the need to scan/send receipt or photo from the ATM (which I take every time, in addition to receipts.)

For Cambodia and Vietnam, there are several banks which do not charge ATM withdrawal fees and these I've been using at Seam Seap and Phnom Penh, also with Schwab card. No problem whatsover.

While I still bring some USD as a cash during my travel in SEA and other developing countries, it is no more than couple of hundred $ and never in my last 5 years of travel I needed to exchange it on local currency.

Bringing several thousand of $ and carry it around to exchange it at the destination country is not the smartest idea from a safety perspective and moreover - from financial perspective. Well, unless you decide to travel to Venezuela. But travelling to Venezuela and moreover, with USD cash is even less than a smart idea.

At the end - to each its own.

P.S. Regarding ATM withdrawal fees - for Vietnam use DongA Bank, EXIMBANK and Techcombank - they do not charge withdrawal fees (my information is from 2016), while AgriBank charges less than $1 per withdrawal. For Cambodia, CIMB, ANZ and MayBank do not charge withdrawal fees (information from 1.5 years ago).