First thing would be to keep it short. Most people reviewing reels determine whether or not to continue within the first 30 seconds. If it's not good, it gets chucked. If it looks worthwhile, they'll watch the rest of the tape. Even then, keep it under 3 minutes.

It terms of an animation reel, audio is secondary unless it's obviously lipsync work. I know several people who watch reels with the audio turned off. Good animation is good animation regardless of the music accompanying it.

If you do decide to go with music, be very careful of your choice in musical style. It's very hard to predict the musical tastes of people reviewing your reel. If you choose some sort of electronica/techno piece and the reviewer hates that type of music, that's already one strike against you (techno was entirely too common in demo reels during the 90s). Whatever music you choose, make sure it is appropriate for the animation and serves to complement it.

Copyrighted music is ok, IMHO, but I would avoid anything that's extremely popular, common, or over-played. People hear those songs and immediately have preconceptions about the song. Again, regardless of the style in music, make sure it's appropriate for the animation.

Hope this helps!