Most experts agree it's best to exchange U.S. currency for foreign currency when you arrive at your destination.  Most foreign airports have ATMs that will convert your money at a better exchange rate than you would get back home.  Even after considering the fees (currently about $3 plus a 1 to 3 percent currency-conversion fee) that you may be charged by your financial institution for withdrawing money outside this country. 

Of course, if you like to arrive with local currency in your wallet, you may want to convert U.S. dollars at your financial institution before you leave.  Most recommend that you avoid the exchange counters at U.S. airports or hotel desks since they often offer the worst exchange rates.