Music and Sound Design is typically part of a larger whole, which you may or may not be a part of, so I can see having it mastered by someone else. But, if you trust another person's skills to market your own, make the distinction that you only composed the music or you only created the sound design. Don't misrepresent yourself to clients. It's bad form, and it will bite you in the ass.
A little anecdote: Not too long ago, a company I was working for was hiring for a position. We actually had two separate individual's reels show up that were exactly the same. They both worked at the same company and rather than compile their own demo reel, they obviously sent in the company's. I have no problem with that, it's work that you did and it's pretty common for multiple people to work on the same project, especially at the same company. But when presented with two reels, from two separate individuals, that are exactly the same, how am I supposed to know who did what? As a result, they both went to the bottom of the list.
Now that everything's on the web (and delivering a DVD seems arcane) I'm a big fan of lower thirds, accompanying text, replaying a sequence with your material solo'ed, personally introducing the clip and talking about it, anything really that distinguishes the work you did from the work some one else has done.