I think when Ms Landers fiance called him Ted, and Teddy, is the only time I've heard him called that.

Ocassionally authority figures would call him Theodore instead of Beaver but I beleive you're correct in saying that Miss Landers fiance was the only one who called him Teddy.

That girlfriend he had in one of the last episodes, 'Beaver Sees America', called him "Teddy."