I know a Kathryn that goes by Kathryn and only Kathryn. I've never seen anyone try to shorten it either. When someone is always refered to as a name, most people tend to just use what they hear.
I agree about using what you hear. My son's name is Benjamin, which I think would fall under the category of names that people would automatically shorten, and even though we call him Ben and never Benjamin, people still ask what we call him or call him Benjamin until told otherwise.