Depending on where in each country you go you should be ok for bank machines, if its gonna be a quick run around Phenom pehn , Siam Reap , HCMC,Da nang, Hanoi etc etc ( on the beaten track) you should be ok relying on ATM's.
In my experience there i would do no harm to have say $100-150 in $5 and $10 bills, v useful.
Enjoy