I don’t know that you are going to find the answer in B&W in a regulation. That being said I would highly recommend that you get those pre-disclosure items back into the loan file. With most regulators if you can’t prove that you’ve done something then you’ve might as well not have done it at all. By getting those disclsoures signed and back in file that provides the proof that you did what you were supposed to do.
Now one of the items I personally don’t keep a copy of and that would be the Privacy notice. We always give them out and when asked about the Privacy notices we always respond with that we always give them out to customers when they apply for a loan and that has never been questioned. Other than that, I highly recommend keeping the original in file.