The Official Folks here will know much more than I do, but as far as I know, you don't have to put a single piece of your prep work here on WDC! You can do the whole thing in Scrivener. (I think that's what I did last time I participated.) The only things you may wish to post to WDC, if you feel so inclined, are the contest entries that ask you to write pre-novel scenes with your protagonist/antagonist/setting and such.

Each week, there's a contest round for those entries, and it's helpful to get feedback on those! I posted mine on WDC when I participated in Prep a while back. I'd recommend it, if you feel comfortable with it. Then again, you don't have to! You just have to do the assignments and then log them in the form. You don't have to put them anywhere else if you don't want to.