When do I solicit the ideas and/or opinions of other people for the sake of plotting? Uhm...I tend to do that after the draft is done, usually when a CP has returned it to me with notes of "Yawn" or "DUMPSTER FIRE" in the margins. At that point, the brainstorming is solving a specific problem. Too soon in the process and I'm paralyzed by the abundance of choice, the internal struggle to make it uniquely mine, plus all the fun of hatching evil plans as I write the draft is gone. I do plot, I do outline, and I do end up with a product that isn't similar to where I thought the story was going.
Don't get me wrong, I LOVE brainstorming. Letting my imagination run wild with "what if" is a blast. You give me "nuclear physics" and I can get us to "were-chickens in outer-space." It's the best drinking game ever. Hell, I like so much, I'll do it sober with very little prompting.
However, when it comes to a story I'm actually going to write/am writing; that's me and my special weird having a grand old time before we expose it to anyone. Once we have, should my Dev Editor or CP make notes of "this is what I think you're trying to accomplish, this is how it reads, maybe try something like this," I'm all Gabrial Iglesias with the giggles, "Yassss, yasss!"
Every creative person has a different point in their process where brainstorming is most beneficial. There is no "right" answer just as there is no requirement that you take any of the suggestions hatched during a session. It's all about when and how you need your leeetle gray cells stimulated. (What? I know that's Poirot not Iglesias!)