I know it's a non-answer but the easy way to find out is to start searching on Ebay on closed sales and see if they are actually commanding a higher price on the market. Search by the part number.
Certain cars and paint schemes do, that's for sure, but I've never seen any evidence of trucks/wheels doing it OTHER that the ribbed-back wheels era being rather distinctive.
I only know one 'collector' that seems to have at least one of everything MT has ever done, dressers packed with cars, but even he has been thinning the herd.
I had one good friend that had a custom run made by MT of 100 34' B&O black hoppers with a unique car number on it, when he auctioned those off about 15 years ago they got truly absurd prices. He was an accountant and said his best investment asset in terms of yield was N scale hoppers. But that was a long time ago.