When decking is installed conventionally, with gaps between adjacent boards, sloping the deck surface is not necessary. Some decking products with limited drainage recommend sloping the deck frame to a 1/4” vertical per 12’ horizontal pitch. This slope is small enough to escape detection by the naked eye, but enough to force water away from the house. In order to do this you will have to trim the structural posts by a small amount to tilt the frame slightly in the proper direction. Your footings will probably sink a little bit too which will add to the slope.