Personally, I think tile and hardwood floors are harder on a dog's hips than stairs, especially if you have a dog that races to the front door when the doorbell rings or if you have multiple dogs that chase each other, sliding around the corners. I would not be concerned about stairs at all. I have 15 stairs going to my 2nd floor, and 17 down to the basement when I let mine out to do their business. Never any issues in 19 years, and I let them go up and down as soon as they are able.
I wonder if the vet has heard about GSDs going over a 1 meter jump or 2 meter wall.