http://en.wikipedia.org/wiki/Affidavit (yes wiki, not the greatest source,lol)
So, if one goes by what an 'affidavit" is, it has to be notarized to be an affidavit!! I just sent letters (not notarized) from a couple of friends. It had there contact info, so if Immigration wantyed to contact them, they could. I sent piles of other evidence as well, so the "letters" were just a nice to throw in item. One doesnt have to send affidavits or letters, all up to you!! With al the evidence I had, i didnt think it was necessary to go get the "letters" notarized. I received my 10 yr greencard with no probs. And yes others didnt send in any letters or affidavits and were improved. Other evidence such as both names on leases, bills, life insurance, are most likely more important. Best of luck, and yes do a search, lots of info.