In most (if not all) states, the supervising physician does not need to cosign all notes. There typically needs to be a plan in place for the "supervision" of NPPs. Something along the lines of reviewing & co-signing a certain percentage or number, or a scheduled monthly chart review, etc. This is determined by state laws and vary. And the law for a PA vs NP may vary as well.
Besides that, some employers may require it, some malpractice insurances may require it, or some insurances may require it especially if billing incident to.
So basically, depending on who is asking, and where you are located, the answer may be yes, no or sometimes.
Hope this helps.