What leveling precautions should one take with plywood subfloor for engineered hardwood nail down installation?
Depending on the size of the sub-floor and what type of material it consists of, leveling is fairly straight forward. If you are leveling a  plywood sub-floor the most critical factor to focus on is the joints and edges of each adjoining piece; they must be level to each other. With a long (48") level and straight edge check each seam screwing down any wayward areas or high spots into the floor joists below. Screw down any sections that when tread on produce a "squeak" or bounce. Use a leveling compound for leveling low spots.