I didn't but I have seen some postings recently that have suggested that. Maybe the regulations have changed or it depends on the area. But back in June 2011 in Paranaquay City, I didnt need a CENOMAR, just my CNI form the UKBA.
Actually I did initially obtain a CENOMAR but found out later that they didnt require it. So it was never used. Only my wifes CENOMAR was used.