I play on Vassal since I have no local LGS so there are some vassal customs that don't have a counterpart IRL. Now technically rolling defense die is not optional but it seems common sense not to roll defense die if no hits are rolled or gotten via focus or gotten by an ability. Is it the practice IRL not to roll defense die if someone whiffs the attack roll? It seems like if someone is rolling defense die they think there are hits that can be cancelled. Once your opponent starts going for his green die it seems like you have time to realize he misread your roll.
I always roll defense die even if there is no hit. It might be with a wink and a nod, but I do it.