I’ve made a dedicated iTunes library for working with FCP. I’ve got all the Soundtrack sounds there, plus several sound effects libraries, bits I’ve made in GarageBand, and so on. I put them all there as 48K aiffs, so they are ready to be directly imported to FCP.
While it’s true that all of these *should* always be available later, I still take the extra step of copying them to the project folder first.