Even in an ideal climate, poinsettias bloom best with some pruning. Susan recommends cutting them back in the "A" months (April and August). That method has worked for her, for years, ensuring that her plant blooms at Christmastime. Your plant will probably still bloom for you, but it might not be during the holidays and the blooms will become progressively smaller or more sparse.
A word of caution when pruning poinsettias or any Euphorbia: These plants secrete a milky sap that can be irritating to the skin. Use care when pruning or handing the plants. Wear gloves, if you are susceptible and always wash your hands afterwards.