Before Blue Bear gets to it, i'll refer to his mixing 101 post, at least the eq tables part of it. (at bluebearsound.com)
To support what the others have said:
When I recorded my friend's voice for a song, once, it came out really crappy, all muddied. He wasn't around for me to re-record it, so I had to deal with what I had. And by raising that 5kHz range, it does add a lot of clarity to the vocal.
Well, that was my first actual helping post.