I too have a dog that doesn't like overbearing dogs. When he's had enough (which doesn't take much), he'll walk away. If the dog follows him and continues, then he'll snap or show teeth. Boxers seem to bother him the most. When he was younger, about from puppy to 1 year, he didn't care who he played with. Now he's very choosy, only playing with dogs he knows, and very sparingly. I was told he's a Border Collie/ Lab mix, funny enough.
I can understand why people would pin BC's as aggressive, they're smart enough to know when other dogs are being rude I think any real aggression would more than likely be a product of the dogs upbringing and/or experiences. (Bad owner, being a stray, ect.) My two cents.