I'd say that posting decks should be mandatory for the Master's Tourney considering that if an illegal deck is used that somehow affects the outcome of the match, then there's no way for you to go back in time and allow the person who should have progressed to play the matches as normal.
With challengers it's not the same issue since everything is dealt with using points instead of a tourney system, but I will say that Trials feels incredibly boring when all you see are the results accompanied by a "will post decks later" that doesn't always happen.