It is common after launching a new instance in EC2 to assign a domain name to it. A useful practice I have found is to create a CNAME to the amazon provided public hostname. This address will resolve to the internal address when inside EC2, and to the public address outside EC2. By using this hostname, you can ensure the best performance and lowest cost by always hitting the server with the optimal address.
This Y-Combinator article <http://news.ycombinator.com/item?id=1460725> says it best. I have used EC2 since 2009 and the on-demand price of an m1.small has started at $0.10/hour. Since the performance of the instance has seen little change, you would expect after one and a half years have elapsed the price to have fallen to 0.05/hour, and after 3 years $0.025/hour. Instead the current price is $0.065 per hour.
When I initially subscribed to EC2 it was a great deal. Now the pricing is becoming excessive. With the advent of OpenStack, I see Amazon being forced to lower their prices.