I've yet to meet someone that will believe their lovable pet is territorial, and would bite a stranger, but I did Land Survey work for many years, and have seen breeds of all sizes come to the door or fence, with hair raised, teeth showing ready to let you know where you stand, then the owner comes to the door, and instantly the dog turns into a tail wagging marsh mellow before it's master see's it's dark side.
Of course, this is when the owner say's "Oh he's just a baby." and chuckles at our reluctance to approach the dog. I've seen this scenario repeat it's self in urban and rural settings over, and over, and over again. Any dog will bite a stranger, that's why the mailman carry's pepper spray. The dog may not bite today, but he may bite tomorrow.
Now before anyone get's all uppity, and thinks I don't like dogs, I'd like to make it clear that I love dogs, have had several, but wont have another one without a fenced in yard to provide it with the space a four legged animal should have.