I don't have an "expert answer" but I have been flying since before I could walk, and I'd say (if they aren't sharp enough to cut) you don't have to declare them but still make the security aware you have them. Just telling them "I have prop knives, they are made of ___ and aren't dangerous." They'll probably ask you to take them out just so they can double check but if you put them on the top of the bag it should be quick.
Check out my work at: jacobcosplay.tumblr.com
Link alt. outfit from "Legend of Zelda: Skyward Sword"
The Every-Doctor from "Doctor Who"
Will debut this year:
Kylo Ren from "Star Wars"
Nora Valkyrie from "RWBY"
Gangplank from "League of Legends"
Cosplays I've worked on for others:
Deadshot from "Suicide Squad"
Zelda from "Legend of Zelda: Skyward Sword"
Weeping Angel from "Doctor Who"