I have seen this done both ways. I have seen at almost all but not quite all casino's because their was no aggressive act on the final street the person to the left of the dealer button must show or muck. And at some but not many casino's the last aggressive act stands as the one who must show or muck regardless of whether the aggressive action was on the flop or turn and the action was later checked on the turn and river. I said muck or show but at some casino's the player must table his cards and can not muck them face down.
The majority of the time I am playing tournaments and most of the casino's I play at follow the latest TDA rules pretty closely however the Beau Rivage was still specifically using the 2009 version of the TDA for some reason last time I was there last year... but anyway
The Tournament Directors Association (TDA) rule 16 states "In a non all-in showdown, if cards are not spontaneously tabled, the TD may enforce an order of show. The last aggressive player on the final betting round (final street) must table first. If there was no bet on the final street, then the player who would be first to act in a betting round must table first (i.e. first seat left of the button in flop games, high hand showing in stud, low hand showing in razz, etc.). Except where house policy requires a hand to be tabled during the order of show, a player may elect to muck his hand face down."
TDA rules link---->
http://www.pokertda.com/custom_posts/view-official-tda-rules/
