That said, people I've discussed screencasting with (nearly all in the Ruby community) have typically recommended video first then a voiceover - this includes Geoffrey Grosenbach of Peepcode (he also shares this technique in his screencast about screencasting).

I tend to do everything in one take then edit out the mistakes so I haven't got an informed preference, though doing the audio first does seem to make sense from a pacing and "maintaining interest" perspective.. you can always edit down/speed up the video parts, after all.

I suspect that merely being detailed enough to think about these processes and how they work (or not) for you is enough to put you above average in the screencasting stakes.

The Rails Tutorial screencasts (http://zfer.us/EKm97) always combine audio and video from the same take (using ScreenFlow, natch). I can't imagine achieving a good teaching cadence any other way. I also love the verisimilitude of hearing mouse clicks and keyboard clacks synchronized with the action on the screen.