I wouldn't say it is a "must" due to the potential toxicity of the wood. If it were, you'd see Echo and RNT doing it, and their tone channels and tone boards are bare wood. Some folks do react to certain woods and i suppose simple contact could present a problem for extreme cases, but the dust is the most dangerous part for most. I oil the inside of my barrels as well as my tone boards and tone channels to ensure that the wood is protected from the constant barrage of moisture it'll be exposed to in use. Some don't, and don't suffer any ill effects performance wise, but I think it is a simple step in an effort to turn out a thoroughly finished product.
I don't CA inside the barrel, and oil the inside of my barrels after applying CA.