I think there is a difference between a redhead and a "ginger". I have been around a lot of natural and unnatural redheads in my life, and most of them have the same personalities as blondes or brunettes. Their hair color didn't automatically change them into violence-prone, temperamental hotheads, like those ginger jokes would suggest. I would say that most of my redheaded friends enjoy telling funny ginger jokes themselves, and aren't especially offended if others tell them.
I will say that there are some people who definitely qualify as "gingers", though. They do get highly offended at ginger jokes that make them sound stupid or volatile or mean. If I had red hair, I'd probably be offended by those jokes, too. I've seen those videos @mrwormy described, and that guy is definitely acting like a "ginger", which I think is actually part of the joke.