Are nails okay for flooring the attic?
Flooring in an attic that's mostly used for storage needs very little in the way of fasteners. Nails are certainly fine, though they're more likely to squeak and work up over time. 

For the few that are needed, and to minimize trauma to the ceiling below, and because I may want to lift the sheets to access wiring later, I'd be using screws. I wouldn't do a full "screwdown", however. A half-dozen per sheet is adequate.