I think that the appropriateness of applying the word "cock" to flaccid penises depends on the circumstances, and is also subject to dialectical variation - in some places I suspect, the word "cock" refers to any penis.

My own usage, and that I've noticed around me, tends to limit "cock" to sexual contexts, though I suppose that if the discussion is of a sexual encounter where the penis in question was flaccid for some part of the encounter, it might still be properly referred to as a cock.