Like LandyVlad, I generally I do a rough edit, voice record, final edit.
If there is something really important I need to say, I may record that part first, work it into the rough edit, then the rest of the voice over, and final edit. I do plan on doing voice overs so I tend to record a little extra footage, to give me more leeway in my talk time. I almost script it in my head as I film, then wait 20-30 seconds before I stop recording. I would rather trim some off than not have enough. It is not like we are using film.