HSC Markers don't give 1 shit about the underlining of texts. Sure its the conventional thing to do to distinguish between characters and the texts title as 51684 said (lol weird username?) but for hsc...you guys have to be realistic when thinking about marks. As long as they can tell its the title of the text they won't care if its underlined or not. That said I do underline mine to make it easier on myself where everything is but I've spoken to quite a few HSC markers and they have all said although its technically incorrect to not underline certain books or magazines or whatever specific ones have underlines and what need " " around it, but they don't take marks away from it...
HOWEVER as you'd imagine if the markers know about it, and your just on the boarder line of a band 6 and band 5 but really unsure, something small like that can knock you down...