Best practices for subtitiling vids?

I'm doing some subtitles for Wiscon, and I'm wondering if anyone knows of guidelines for subtitling vids; i.e., is it best to start a subtitle a moment (how long of a moment?) before they lyric starts, or right at the same time? What about ways to get the subtitles to flow best with the music, or the vid itself? Is it better to keep a subtitle up a few frames longer than necessary when there are brief pauses in the lyrics so that you don't have a stutter effect (a few frames with no titles between two different lyric lines), or is it better to match the timing of the titles exactly with the lyrics?