I have written a ltter with every Offer I have submitted, (every!)
I think it humanifies the Buyer to the Seller,
I think it is a class thing to do, (letter writing used to be a classy way to communicate)
I think it opens doors, in the process, that should be opened and oiled.
Always write a letter from the heart; My Clients are great people, they LOVE your house, they would like to raise their family here, etc.
Good luck and may God bless