As already mentioned it all depends on the age / breed..
I have an 8 month old German Shepherd / Chow mix, and a 5 month old Black Lab. Both are protective, but my Shepherd is more-so.
My Lab will bark when he hears a different voice in our house, and he gets pretty alert when cars pull up in our driveway. But he's very friendly with any people that he meets.
My Shepherd will bark / growl, basically show some protective / aggressive behavior when she sees someone in the dark she doesn't recognize, or when she senses I don't like someone (animals, especially dogs are VERY in-tune to your energy, and how you feel every second). For instance, my mothers ex-boyfriend stopped by the house, and instantly she (my shepherd) knew I didn't like him, and she began expressing protective / aggressive behavior. And with the people I like, she likes too.
Caution though: if you plan to allow your GSD to express aggressive behavior (*at any time*) when people arrive, be darn sure that your dog/s understand you're the pack-leader, and you must feel 1000% confident you can control them under any situation. You don't want Roy attacking someone by mistake (obviously. lol), so again you must feel completely confident in your role to control that situation if it happens, before you allow / encourage him to be protective.
This is just my advice; I'm merely a Dog Behavior / psychology 'apprentice', but I like to think I can give decent advice even though I'm not an "Expert", yet.
Also I don't know your level of experience with dogs, so forgive me if my info was a bit patronizing.
Have a great day! ~Rob