Parashath Noah, Laws of Ziziyoth, Page 15:
7. The corners of the tallith to which the ziziyoth are tied must be square and not rounded, as a round corner does not meet the specification of a corner as defined in Halachah. It is particularly important to watch out for this on a tallith qatan, as the corners are usually sewn, and can easily become rounded in the process.
This learning should be in memory of Maran, HaRav HaGaon Ovadia Yosef, ztz'l.