If you're going abroad, getting local currency from an ATM attached to a bank is cost effective and easy, but the problem is that you only get large bills, and many times the attached bank is not open to exchange for smaller bills.

When traveling, especially in South America and Africa, I have found it helpful to bring American one dollar bills for tips. There's nothing worse than trying to ask someone who you want to tip to make change.

Dollar bills are almost always greatly appreciated, and are typically very tough to find once you're abroad.

Counterpoint: Don't pay in dollars - International travel tip