American English speakers in different regions pronounce certain words differently. Actors and other speakers in the public eye often choose to cultivate a style of American speech unassociated with a particular region. Dennis Haysbert, for example, speaks a careful and elegant style of American English that does not reflect any particular region. Fran Drescher and Lucas Black, on the other hand, choose to make their regional accents a part of their acting personas.
Minor differences of pronunciation are not usually a cause of misunderstanding. Some pronunciations, however, can result in misunderstanding or misspelling.
A frequent difficulty in understanding the speech of others is not so much pronunciation as enunciation: the careful uttering of speech sounds. People who speak quickly and slur the syllables in their words can be difficult to understand. A good way to practice both pronunciation and enunciation is to memorize and recite poetry.