In short:
No. Not really. You don't need to do use them either, because RSS Graffiti aggregates your feeds for you.
Technicaly speaking:
Aggregate feeds don't always behave like a stream (a chronologicaly modified series of elements). The sub-feeds contained in them don't usualy have the same refresh rates. Nor are they updated as often. This, often results in newer posts from sub-feeds appearing in the aggregate feeds later than older posts from other sub-feeds. RSS Graffiti relies on the date on which each entry in the feed was published to determine which elements are newer than those already posted to Facebook (and thus should also be pushed to your Facebook Wall). So, some entries that make it into the aggregated feed later than others might be considered as old news and never get published to Facebook.
RSS Graffiti might use entry GUIDs in the future to determine published entries from unpublished ones. But this has not been implemented yet since it would require an amount of resources that is currently not justified by the needs of our users or the current scope of this application. Nevertheless it is an improvement that we are considering for a future release.
What to do?
If your aggregate feed misbehaves in the way described above, then consider entering the sub-feeds directly into RSS Graffiti and let RSS Graffiti aggregate them for you. This should work like a charm.