Large dogs that are active will use their nails to dig into your floors to get traction. This could scratch the surface of your hardwood floor. If your dog will be sharing your floors make sure their nails are trimmed neatly and they have are rugs that they can use to get traction to stand up. Make sure that your floors are regularly maintained to replace the surface protection so they baselayers are not scratched.
If your dog is not house broken, do not let your dog near hardwood floors. Pet urine will etch the flooring with the acid it contains. If the urine is not cleaned, it will darken and change the color of your floor.