The fact of the matter is that a vast majority of dog breeds have tendencies towards several types of aggression. Some breeds tend to be wary of strangers and fiercely loyal to their owners. Without socialization, this wariness can often turn into stranger aggression. Example: Anatolian Shepherd. Some breeds are born with the innate susceptibility towards dog aggression. One obvious example is the Pittie. Then, there's also Akitas and Tosa Inus. Then, there are dogs who have are incredibly headstrong and have natural tendencies toward resource guarding. Without training, that turns into resource aggression. Example: Shibas.

It pretty much goes without saying that the common denominator in aggression prevention is training and socialization. With a heavy emphasis on these two aspects of dog-life, you can curb most aggressive tendencies in your dog. To say that all terriers are quite aggressive is a broad brushstroke that does not take into consideration the massive role the owner plays in a dog's behavior.

Jess, I know you are very experienced with dogs and their training. I'm sure that you will do fine. The problems only start to arise when people ignore problems and refuse to train/socialize their dogs. You'll be fine with a Bullie