Sabrina

I, also am a rising Robertson at UNC. I did not fill out the supplemental, and I feel like that definitely helped me. For those interested, students who did not fill the supplemental out in October or November are instructed to complete it in the month between being chosen as finalists and the finalist weekend. My responses in March were dramatically different (and I think much more well thought out) than they would have been had I filled the supplemental out in November. My experiences between October and March changed me a lot, and so I responded differently, addressing different topics. The responses were definitely more current, and thus easier to discuss in my interview. As far as I know (and I may be wrong, since this isn't how I did it), students who complete the supplemental in the fall are not given the opportunity to edit it in the spring before the finalist weekend.
If your application is strong enough to be considered for the Robertson, I would wait to do the supplement in the Spring. Again, for those interested, I was admitted to UNC early. I sent a midyear statement along with my midyear report including recent developments (national merit finalist, other awards and projects, etc) and stating that I wished to be considered for a merit scholarship. The applications to UNC and Duke are really the applications to the Robertson, so I would focus on making those the best possible and then consider the supplemental later. Also, remember that the Robertson is service/ leadership oriented. Use the regular application to your best advantage, and then use the supplemental to address activities/ accomplishments/ traits on which you haven't yet been able to elaborate. The interview committee uses both in the final interview.
Sorry if this didn't really address the initial post, but I thought I'd share my experience with the Robertson selection process for rising seniors.