_domainkey.yoursite.com TXT "t=y; o=~;"

Does anyone know how necessary this entry is, as opposed to just having records beginning with selectors?

It seems like the t=y means that testing is on and to not actually block messages that fail DKIM, and o=~ means that some messages aren't signed. I'm not sure why the article is suggesting people use those settings, since they are entirely variable between different users and their config.