Ok, well lets keep in mind there will be some floor joists going in to support the deck, so when the deck goes on, if the floor boards are allowed to overhang the outside edge of the frame, they will be able to be trimmed in order to achieve a square cornered square all around.
That said, and I know I'm a bit late on the uptake, but perhaps the next guy might be able to use the information in this picture before they start to build on a slope.