there are certain breeds i'm certainly more cautious with until i get to know them, especially in the vet hospital environment- chows are one (though the only one i've seen at the practice where i work now is a big squashy love), shar peis, springers, and most adult intact male dog with some macho clueless jerk holding the leash (or the chain, in some cases).
interestingly, none of the dogs i've been bitten by are on my list of breeds to be cautious around.