As I said, all T-Mobile plans, prepaid or postpaid, include domestic roaming at no extra charge. Whether you can actually roam in any given area is another question. There are a lot of places where T-Mobile does not have native coverage, yet you still cannot roam on other networks, and your phone will display the message 'emergency calls only'. In that case you can only place 9-1-1 calls.
Edit: See
T-Mobile coverage map.