I'm Katharine on my birth cert, but was called Katie until I started school. There was another girl in the class who was just Katie on her bc so ended up as Katharine in school! By the age of ten, got fed up with having one name at home and one at school so now always use Katharine.
Another thing to watch with the full name is the spelling - Katharine,Katherine,Kathryn, Catherine etc, etc! Though I now quite like the more unusual spelling of my name, can be a bit of a pain having to spell it all the time. Also causes problems in e-mail address as people always asume it is with an 'e' in the middle!!