Further, some may argue that authentication and registration means the same in general practice but arguing contrary, if notarisation is an option, than the issue is solved.
Hence, PoA has to be registered only if a person (who is a PoA holder) presents a document for registration as an agent of another, then it such PoA holder should have it authenticated/notarized.
And conclusively, Registration Act lists out all the compulsory and optional documents to be registered under Sections 17 & 18 respectively, where PoA is not mentioned at all, hence no general PoA can be said as required to be registered.
p.s. Some states (like Tamil Nadu) have made irrevocable PoA to be registered..