It depends on the circumstance.
Coming out to people who know you love you, and are loyal to you, educates them, which allows them to be good allies - influencing those in their lives who my be hateful. They unfortunately have more influence than we, as transpeople might, because they receive more respect (generally) as cispeople.
Coming out to a lot of people who couldn't care less about you might educate them, but their pre-existing prejudices get in the way.