I have a theory that aggressivness in cockers is something which gets a lot of publicity due to a wide variation in what they look like and what they are. What they are, are dogs, decended from canis familiaris like all other dogs, including Bull Terriers!
Example you are walking down the street with a Bull Terrier, a young mother and child approach, she takes hold of her child and gives you and the dog a wide berth.
Now walk down the street with a cocker and the same young mum and child appraoch, this times theres lots of "ooh gorgeous doggy" reach out and give it a hug / stroke. The dog panics and takes a nip, suddenly everyones shouting about rage! (which is a form of epilepsy, and not a behavioural condition).
What I'm getting at is because of its looks (cute), then people tend to forget that its still a dog, and not a real life teddy bear. And so when it acts like a dog (and not like a teddy bear) then suddenly theres a big gap between what people expect its behaviour to be like, based on their assumptions made on the dogs appearence. Thus a big deal is made out of its "aggressivness".
Any dog that has, as broad a mix of blood lines as possible stands less chance of been affected by any hereditary conditions, ask a vet which dogs have the least problems and S/he will probably say mongrels.Optigen testing for PRA prcd, is more of a concern for prospective owners than worrying about rage.