You have to be very explicit when agreeing on rules. Backpocket probably more than any other game has to have ALL the rules you are playing by established before you begin play! You WILL run into rule problems otherwise.
Again, the rules can vary between different areas and different opponents. Make sure you always agree to the full set of your rules.
One thing you have to watch out for is the coin designating the back pocket. Its usually alternated each game, like in 1 pocket. However, you MUST stipulate that if the players forget to move it, it can NOT be moved once the rack has been struck on the opening break. It stays where it is at that point until the next game. This will save you tons of arguments during the course of the game when the 9 moves to a position that favors one of the pockets.