The Onigiri (Omusubi) FAQ

In the subject FAQ, you say that omusubi can be any shape. I am a fan of NHK Taiga drama and I noticed that during the Sengoku Jidai period, the warring states era, that the omusubi was round. In Hawaii where I am from, round omusubi was only served at funerals and never any time else. And fell into the category of only at funerals. Like passing anything from chopsticks to chopsticks. I have also heard it said that triangular omusubi represented ten-shi-jin, heaven-earth-mankind. Any information you have on this aspect of omusubi would be appreciated. :) ありがとうございます。 :P