In my past life, when I worked for the Alberta government, my wife was an auditor, and she sometimes was asked to audit some of the agencies that I was responsible for. Ethics screens had to be set up for both of us, so I understand being covered by an ethics screen. You have to ask a lot of hard questions about the specific situations. It's hard to generalize about them, but in the end, I think it's up to the integrity of the people involved to make sure that they're trying to make decisions that are as impartial as possible and are in the public good.
You have to be able to defend your actions as being ethical and in the public good, and there isn't a perfect cast-in-stone solution. It depends on each individual situation.