The best way to fasten the decking in new construction is to use tongue and groove plywood. Glue it to the floor joists with decking adhesive..... Then nail or screw as you mentioned. If this was done, no further fastening should be needed. Adding extra screws won't hurt a thing though. Length of screw depends on thickness of the decking. We use 2" "deck" screws for 3/4" material. BTW If you already have squeaks be sure to drive the extra screw next to the existing ones about an inch away. This keeps the decking from moving on the nail thus causing the squeak.