Never

I think the question is in relation to Heads Up play

Pushing is done when you are first to act or to negate bad position after the flop. However in HU First to act Pre flop has position after the flop so I think pushing is bad here unless a flat call leves u pot commited 2BB but also watch the size of there stack. Sometimes both players will be short stacked in relation to the blinds.

Then pushing becomes the default play as Tourney is now a crap shoot and skill will have no bearing on the outcome.

In HU your are getting better equity flat calling in SB rather then pushing.
Why Push from the BB when u can see the texture of the flop for free and then push to steal if you so wish.