Roaming just allows you to receive data/voice from other carriers for which Sprint has agreements with. They don't want you to roam because it costs them money for you to do that, especially if you do it a lot. I would enable it so you can receive data/voice in areas with lacking coverage. Don't worry about how much you roam until Sprint notifies you. It usually requires several tens of GB's, regularly, to receive a notice from Sprint informing to cease.