Hi Nick. As I interpret the rules, it is entirely within a TDs prerogative to require any specific cards to be shown at showdown, including the winning hand, or even all cards if they so choose because the rules provide:
"...Except where house policy requires a hand to be shown or provides an express right to see a hand on request, asking to see a hand is a privilege granted at TD’s discretion...."
So, if you as TD declare house policy to be that all cards at showdown must be shown, that's within the language above.
Obviously there is a large contingent that does not favor requiring all hands to be shown, for various reasons. So there are at least 2, probably 3 camps on this issue, the language above permits all to co-exist within the larger tent of the TDA. What is certain is that TDs should make their specific house policy clear on the matter so that players know what to expect.
I'm sure that which hands must be shown / may be shown / are a right to see on request / are a privilege to see / will come up again at Summit VII, but from what I've observed it really gets to philosophy of the game, and this is one of those issues where we may never have a super-majority, much less 100% agreement, hence the rule has to provide latitude.... A GREAT DEAL of progress was made on sharpening this and other issues pertaining to the showdown at Summit VI, IMO some of the most important results of the Summit.