When?  Whenever!   Whenever you wonder whether it's a good idea to go alone with Ace, King, Queen and 2 outside Aces.  Whenever you want to figure out whether it's best to lead the Right Bower or and off-suit 9. Whenever you want to try defending alone against a loner and you never seem to get the chance in a "real" game.   Whenever you want to see how many times out of a 100 the opponents would get dealt a guarded Left Bower when you're dealt the other 5 trump.

Heck.  Maybe you're just tired of 9s and 10s and have the urge to play a "perfect" hand for a change.