The link between breeds and aggression is tricky for a few reasons...
The more popular the breed, the more likely you are to see aggression cases. This should be self explainatory but is often overlooked when quoting which breeds are more likely to bite etc. It follows that the more common the breed, the more likely behaviourists are to see that breed for aggression problems. This is why the most common breeds seen by behaviourists include popular breeds such as labs, GSDs, border collies, springers, cockers, staffies...
However, there is some variation between breeds when you look at the types of aggression. For example, the APBC reports more cases of border collies agressive towards humans they know that towards unknown dogs. However, GSDs are more likely to be aggressive towards unknown dogs than known humans.
Part of this links to a dog's socialisation window. Some breeds (typically guarding beeds) seem to have a shorter window when they are puppies in which to 'get used' to the types of indiviuals they encounter than other breeds do. This makes sense. If you have a guarding dog you need it to learn it's family/flock quickly and then be suspicious of anything else.
So, you should meet more GSDs that are aggressive to your dog than some other breeds right?
Or not. Because a GSD is a big dog. Therefore, aggression in GSDS is LESS likely to be tolerated by humans. An aggressive GSD is more likely to be surrendered or rehomed than an aggressive chi, for example. So, does that make it MORE likely you will only ever meet sweet natured GSDs? There are plenty of them.
Life does not follow statistics quite so neatly. I guess what I am trying to say (in a roundabout way) is you cannot use those kinds of facts to shape any worries about a specific dog you meet.
Instead, good knowledge of dog body language, good safety measures and using the other owner's behaviour as a guide all helps.