I think the best place to introduce backstory is after the audience no longer thinks backstory is necessary. This goes back to an interview you referred to in an earlier post--tease the audience by refusing to tell them what they want to know when they want to know it.

Incidentally, introducing backstory to the audience is different from introducing backstory to another character. Character X only reveals her backstory to Character Z when she trusts Character Z. If the writer wants to present the backstory as part of a confession from X to Z, then the writer has to build the trust arc between X and Z first.