Luna (Akita/GSD mix) started exhibiting guard behavior at just shy of 4 months. She barks/growls when she hears someone coming at the door, and then if it's someone she knows she stops barking. We did have one instance where she immediately took a liking to one of my friends (looked/acted similar to me) and stopped barking, but otherwise she doesn't stop until that person has proven him/herself to be a friend (we have to work on getting her to stop barking on command).
We're pretty much socializing the heck out of her... puppy class, inviting groups of people over, going to other people's houses, etc. and she still has that instinct to alert when someone comes onto "her" turf.
I don't know that she'd ever attack, and that's okay. I think just having a big dog barking in an intimidating fashion is enough deterrent.
I don't know that I'd ever want to be responsible for a dog trained to attack. I don't have the knowledge/know how and it just seems like a lawsuit waiting to happen. Most (if not all) lawsuits involving a dog bite/attack end up with the dog getting euthanized, and I'm not willing to trade Luna's safety for my own.