I'm not involved with this project so take what I say with a grain of salt. From what I've seen with any mod involving them, voiceovers should always be done last. Two points to consider . . .
- Lines change often: Volunteers that record one set of perfect recordings maybe not available down the road so it's best to record a part's lines all at once. Since dialogue (followed by voiceovers) tends to be the last thing to be finalized, that's why recordings tend to be done best when done last. Keep in mind, you might be available all the way through but I'm sure you are not the only part or actor under consideration.
- They aren't as important as the other parts of the mod: If you don't have everything else worked out in the model - cutscenes, character movement, etc - then voiceovers recorded early will emphasize that things were rushed.
TLDR: Think of voiceovers as the cherry on top of the ice cream sundae. You have to make the rest of the sundae before you put that cherry there.