First, I agree with Alan that using Nikon authentication is a good idea. The software required to read Nikon authentication is a few hundred dollars, but that expenditure should fall to whomever challenges your work -- not you. Now, since the .nef is what will be authenticated, be prepared to provide both the raw and the jpg versions. This will give comfort to the other side, so they can see that the autenticated .nef is not different from the jpg. In that way, they can prove the photo is real and not photoshop. Last, I'd advise keeping your files for at least 3 and maybe 5 years. If you are licensed as a professional in any field related to foresics, investigations or photography, check the applicable ethics for any required holding periods.