I think I'd install one at the top as well, just to play it safe. I'm assuming that stairs become safer (but not ideal) once pups are fully grown- so at about a year old?- which is also when it seems that you can start to think about agility classes. Until then it must be best to avoid any unneccesary pressure on their bones and joints.
Having said all that, we're currently doing major battle with Barkley (just coming up to 4 months old) over the sofa and it's not easy to stop him jumping off, especially as he hates us picking him up and placing him on the floor. I cringe every time he manages to jump, especially as lately he's taken to actually leaping off, almost as if to make a point!