i ride the guagua between santo domingo and boca chica and the taxi publicos around sosua, alone, all the time when i'm in the DR. never had a problem. similarly with regular taxis where ever i am in the DR.
you must know what the fare is and ensure that you know where you're going but i find that almost everyone is quite helpful.
i make sure i know the correct fare, as mentioned in the thread, by asking either friends before i go or others waiting, and just hand it to the guy when asked.
just be sure of the details and be friendly yet confident.