Hi Bart,
I couldn't say what Mr. Suzuki was specifically referring to and have seen very few rectangular stands with rounded corners. Surely, the basic thrust of his points was that some trees are so feminine that the masculine or harsh corners might present an uncomfortable conflict in the overall image, but this is just my guess. Sorry that I can't really speak to his idea.
Kind regards,
Andy Rutledge
www.bonsai365.com/
zone 8, Texas