I work with dogs periodically that do not get along with the dogs in their own house. Sometimes the first dog has been fine with every dog previously, and then there are issues with the new dog that comes home.
Ever see a dog growling around his food bowl? Ever see those "Funniest Home Videos" shows where a dog is showing teeth while standing over a bone? You have witnessed something called Resource Guarding, a form of dog aggression. This form of aggression is fairly common, but easily preventable. As with all undesired behaviors, I always recommend investing the time in the beginning to prevent a situation from arising, rather than having to work on changing behavior later on. Especially with aggression, once a dog bites someone there is always a chance that he might bite again.
There are a few preventative measures you can do with your dog to lessen the likelihood of problems later on. Unfortunately, even if you have the best intentions and do all the exercises properly, there are no guarantees of eliminating the problem later on. Regular maintenance is also a good idea.