Hi,
Speaking from my own experience, I do think that a notary is required. When we completed the I-864 we had the notary do the stamp at the very bottom of the document in an area of white space.
I would strongly recommend that you contact the appropriate people before sending your forms in, just to verify. It could save any unnecessary delays. Maybe things have changed since my I-864 but there's no harm in confirming before you send your forms.
If you're sending your forms in to the National Visa Center, I think that it's best to email them. They usually responded to us in 3 or so days.
Best of Luck,
Scouseredneck