I think it has to all be from memory. If a Scout wants to work with me on a skill, they are more than welcome to use the book. We learn together, we use the book as a guide and aid. That doesn't count for a sign off, though. Come back the next day, demonstrate the skill without an aid, notes, etc, then I will sign it off.
This is why I find "merit badge days" so hard to staff. It is hard to do that without requiring pre-reqs that are actually done.