Don't worry about the protectiveness, because that part is natural to them. What you want to achieve is the well balanced German Shepherd (the type that doesn't lead to a law suit).
A well balanced GSD is friendly. That's a good thing. They should understand who you suggest is good or bad who enters your home.
With that said, my German Shepherds never showed aggression, or protectiveness... that is, until they were put in a situation that showed you otherwise. We never had an intruder in our home, but there was a time that my dad came over for a visit and he just opened the door (without knocking and being welcomed in). Our GSD, who met him nicely previously, said NO. He quickly shut the door and knocked like everyone else and all was good when I welcomed him in.
Oh, and I'd say that GSDs don't hit that protection maturity until about 2 years old.