There is a good chance that if you are asking this question, the answer is YES! Why? Peace of mind and you never know when it will come in handy.
There certainly are countries that require you to carry an IDP (international driving permits) in addition to your drivers license, and if you are caught operating a vehicle without one, you may be fined. Don't think that because you are in another country, the charges won't follow you home. The rental agreement that you sign upon picking up a car likely gives the supplier authorization to charge your credit card for traffic violations incurred under your name.
Basically, the IDP translates your driver's license info into several languages. While an internet search may pull up a number of companies that all claim to provide the official documentation needed for the permit, you need to be careful about potential online scams.
In the U.S., the two organizations that have the Department of State's authority to issue IDPs are the American Automobile Touring Alliance and the American Auto Association, whose members can also get the best car rental discounts through Auto Europe. Canada's official agency is the Canadian Automobile Association, and for a list of valid locations of where to obtain these in other countries, click here.
You must hold a valid driver's license from your country of origin. An IDP is to be used in conjunction with your driver's license. An IDP typically costs $15 USD. The IDP is effective for one year from the date it is issued.
At the very least, an IDP will save you and local authorities time if you are pulled over in a country where English isn't the primary language spoken. For the best guaranteed rate on your rental car in Spain, Australia, Mexico, or anywhere else in the world, choose Auto Europe.