Is a NIOSH mask advised when sanding a fully dried hardwood floor?
NIOSH (National Institute Of Safety and Health) does not manufacture respirators or dust masks, they conduct research and make recommendations for prevention of worker injury/illness. If their name is on a respirator it means they approve of it's use for the manufacturer's stated purpose. 

Generally. you should wear a particulate mask or respirator when sanding, mostly because of the chance for lead in old coatings. For old varnish the chance of significant lead exposure is slim and no volatile organics will be released.

However, you should wear a N99 dust mask or P100 particulate respirator anyway because breathing airborne dust when you don't have to is dumb. Ever blow your nose after working in a dusty field? Do you really want that stuff in your respiratory tract?