My opinion is similar to others, I only letter pieces that I am curious about and for no other reason. If the information that you have is enough to satisfy your curiosity, fine. If you are leaving everything behind for your son who has no real interest in what you are leaving, fine. Satisfy yourself, in the end it doesn't seem to matter.
If on the other hand you want to make it a little easier for your heirs to more easily get top dollar for what you are leaving behind then a letter surely can't hurt. When I am looking at a purchase of a "collectible" piece a letter works towards the positive if the price is right. A letter will cost you $75 today and to its credit will certainly increase the value of the piece accordingly, in much the same way as matching stocks will and for that matter a matching box, which is really nothing more than the box that it was possibly shipped in.
Without a direct provenance of proof everything is just speculation, curators can pick apart what might be considered a rock solid provenance, right down to whether or not the signature is fake or factual.