We've traveled quite extensively and have never had any problems with the local currency. Obviously, it depends on where you're going. For example, if you're going to Europe, alot of countries use the Euro, but Switzerland, Czech Republic, and England are some that do not, along with several others. So it's good to do your homework.
If we're going to be in a particular country for several days, I'll order currency from my bank ahead of time so I have cash in hand as soon as I arrive.
We just got back from a 25-day Asian odyssey and we used 5 different currencies; Hong Kong dollar, Taiwan dollar, Japanese yen, South Korean dollar, and the Chinese yuan.
Some places will accept U.S. dollar, but alot of places will not, so it's best to assume they don't.
Best thing I can advise is plan to use your debit card at ATM's, which are everywhere, to get some local currency. Use your credit card for major expenses.
Just make sure you call your credit card and debit card companies to let them know ahead of time where you'll be traveling. Plus, unlike in the U.S. where you have a choice of withdrawing cash from your savings or checking, in other countries, when using your debit card, you can only withdraw cash from your checking account. So plan accordingly.
But one thing we've learned in all our travels, as long as we have our debit card and a credit card, we've always been fine and never had problems. Just a few weeks ago we were in China and used our debit card to get some Chinese Yuan.