The point of communication is to be understood. More people will understand what you mean by saying "Kiki's Delivery Service" rather than the untranslated name in Romaji. I'm a huge Ghibli fan, but I've never heard "Majo no Takkyuubin". I know 'majo' means magic, but that's it.
If the person you're talking to knows it by that name, then go ahead. If you're tweeting to a larger audience, it's better to just use English.