You can also often get better prices in shops if you pay with greenbacks. In general, cash is better than traveler's checks.
Cash can be easier and quicker to exchange and command a higher rate or the commission can be lower.
In some places, the larger the denomination of bills, the better the exchange rate. Refer to your guide book on the existance of any black market for currency exchange and whether to or how to use it. Some countries are more tolerant of black markets than others. You may not be able to exchange money at all in small towns or in the countryside. Money can also be exchanged in many hotels.