I have a Diamond HFV-5 (rotatable dipole) in the attic fed by ~30ft of coax. The antenna system comes with a Balun at the feedpoint. Would it be worthwhile to add an inline RF Choke (Unun) in addition or is that covered by the Balun?
A balun effectively allows you to connect a balanced antenna (such as a dipole) to an unbalanced feeder (such as coax). You have a balun at the feed point of the antenna, with a coax socket on it, which means that the feeder going from the radio to the antenna is unbalanced.
The HFV-5 is an antenna that presents a 50Ω impedance on its coax socket and it already has a balun. Your radio's impedance at the antenna socket is presumably also 50Ω, and I am assuming that you are using 50Ω coax as the feeder.
I am puzzled as to what good the unun would do in this system.
(So to answer your question - I don't think it would be worthwhile to put an unun in addition to what you already have).