I realize this is aimed more at unofficial, online VAs, but surely the majority of us don't do this just for fun and want to get paid someday. I know I'd like to try, after I get enough practice in this area of the field.
I attended a panel of fairly well-known VAs at the last Dragon*Con, and one of them (I think it was Vic Mignonga) said you should not show off your best imitation voices for potential employers in an audition or demo. He said something along the lines of, "Your Homer Simpson impression may be spot-on, and that's great for parties, but it's no good for getting a job because we already have someone who does that voice.".
If you want to make fandubs of animes or video games, should you specify that if you are able to accompany any text with your mp3 (such as a YouTube description)? I have a voice I want to use some time that was inspired by the Warcraft peons, and of course I'll use it around my friends for quotes, but should I read unrelated dialogue with the voice in my demo so people won't think I'm just impersonating a peon? The text made it sound like it's OK to do a Sonic the Hedgehog voice or some other popular character, provided you speak an interesting or emotional line with that voice.