Depends on priorities of your BOSS. If he is not accountable for task A, he might never ask you. If you and your boss work in same location, it is very easy to judge.
Typically I will not do anything that my boss has asked me to do unless I know that it is his accountability and he looks good (for his boss or client) for doing a task. And if I dont like something, I either Deny or Delegate or Delay but never Do.