I believe most, if not all, of the NPC organizations allow their new members to wear letters. The only one I have heard otherwise about (from a reputable source
) is AEPhi. There might be a few others, but I know that most of them do allow new members to wear letters before initiation.
I think a lot of chapters are just confused about their national policies. Some of them may be repeating rules from whenever they were in schools years ago, and the rules may have changed since then. Or they just might not know -- I remember a post in in Pi Phi forum where some of the girls said that Pi Beta Phi didn't allow new girls to wear letters, and then a national officer came on and said, "Actually, we do -- and if your individual chapter doesn't, we consider that hazing." (I am not trying to single out Pi Phi here because there are posts from members of ALL organizations who aren't sure whether or not their organization allows them to do this.)
Tri Delta does allow its new members to wear letters before initiation.
I think a lot of local and multicultural organizations are different, too.