RULE does state that the catcher may not be out of the box prior to the release of the pitch.
RS for FP makes an allowance that if the batter moves up to the front of the BB, the catcher may move closer to the plate. Please note, that means that the batter must be in the box and "set" before the catcher can move up. If not (and this is something I would do because I like playing mind games), I would always enter the BB in the front and then step back when the pitcher steps on the PP.
Personally, I don't care where the batter is and neither should the catcher. This allowance really doesn't make any sense since the pitcher should be throwing to the strike zone, not the batter. Yeah, I know the pitcher may try to fool the batter with a pitch that ends up in the dirt, but that is the battery's problem and shouldn't be supported with a special interpretation. But like I said, that is purely a personal opinion, not that of the rules.