I think it's ok as long as your bird understands and listens to you when you ask to: Step up, Step down, as well as he should have to follow the rules of NO nibbling, playing with jewelry, clothes, ears etc.
I allow Dalia on my shoulder, She is there as I type this... Only because she listens and follows through when I ask her to step up. Also follows the "rules".
She knows she is not supposed to be playing with my earing or ear etc but, she breaks the rules once in while. She is then taken down from my shoulder and placed in my lap or on her playstand.
So It really is up to you though and has a lot to do with wether or not you feel comfortable having your Grey there.
B) Cristen