It's really very simple.
Piit beat NE, so Pitt wins the tiebreaker. That means if they have the same record Pitt really has the equivalent of a one-game lead.
Going into the last weekend, if Pitt and NE have the same record Pitt has to win to keep the one-game lead. If Pitt is one game behind they'll be playing to catch NE and get back the tie-breaker advantage. If Pitt has a better record, the Steelers will have nothing to play for because they'll have the equivalent of at least a 2-game lead with one game left.
So all the combinations come down to whether NE can have one more loss than Pittsburgh going into the last game. And they both play at 1 p.m. on Jan. 2. Since NE has San Fran at home, I expect the Patriots will have a big lead early so the pressure to win will stay on Pitt if they start the day with the same record.
Possibly the best chance for the Bills (with Pittsburgh losing to the Giants at this very moment) is: Pitt loses today, goes into the Bills game one game behind, sees NE piling it on vs. SF on the Ralph scoreboard, and relaxes.