Is human aggressiveness okay in any breed? When, if at all, is it acceptable?

I own a 10 month old American staffordshire (pit bull). BYB I am sure. Was Given to me as a young puppy. Although she is loving with family & people she knows even down right submissive. Lately she has been getting aggressive to strangers. The other day I was parked in front of a yard sale and she was in my car and a tweeker got real close and she flipped out growling & snorting & Jumping towards the window which was partially closed. But this has not been the only time. I am afraid I will have to have her put down if she bites the wrong person. I live in a very seedy part of town so its good to have a dog like this in the yard. No one will try to enter. I know its not typical of the breed. But what are your thoughts? Yes, American staffordshires ARE considered pit bulls in this area. She is very good on walks. But that may be because she is in unfamalair territory.