Unless you are only going to do business with/socialise with Americans, listen to as many English speakers as you can.
I've found some Americans can't understand any other accent than their own. I even saw a site where Americans listed which accents they could understand. Some of them couldn't even understand people from other parts of America. Learn English from American sit-coms, soaps and televangelists and you'll be able to communicate with these people just fine .
I was just joking about the last part .