I'm not 100% sure about this since I don't use text internationally. However, I believe that as long as you are remain in the US, sending and receiving messages uses the standard rates (10 cents each or part of a bucket you buy). When you are in another country, it's 35 cents to send, 10 cents to receive.
I've previously sent both text and picture messages from Canada (while roaming on Rogers) to the U.S. and the messages went through fine.