I think Taka is good if you're a Kannagi user. Otherwise, Heishi can utilize Ochu (capped) or Kanaria with TA/STRorDex/AccAtt augments. I guess if you have Taka and no Ochu/Kanaria, its fine. It has agi and ranged acc which is great for Daken, and its well-rounded. The AGI won't help your WSs much with Heishi though.
I think the Ru'Aun katana is sufficient, but it can be altogether skipped if you get one of the two I mentioned above. I rank it as kind of weak, wouldn't go out of my way to get it.
The new conversation will start revolving around these ambuscade arms, specifically the dagger, katana, and sword. I'm sure those more knowledgeable at number crunching will be able to give some hard details as to which weapon is the ideal offhand for Ninja. I have my eye on the dagger for now.