WHY would you want to ?
With 45 seconds left to play in the RWC final, having just scored a try to move 4 points behind, it sounds like a +EV decision to me.
if they had in-goal laws correct as well as touch laws correct i'd be all for having the option to turn down the conversion... but it is way down on my wish list of things that need to be fixed law wise to make the game playable