Not answering for JTrott, but in my experience...
Rats are a social animal and what you put into them is what you get out of them. I've not come across a single naturally aggressive rat in the 3+ years I've been breeding them. The bites I have received(very, VERY few) are most likely from individuals that I have not handled as much, which are more nervous and fearful...not mean and aggressive. The reason I feed off rats that bite or cannabalistic rats is simply because I do not want to keep rats that will bit me or eat their babies. For me, it is not breeding for temperament, since they already have such a good disposition naturally.