Should I know how to use a knife?
That depends. Are you trying to learn how to use a knife?
Not really, but my girlfriend is.
If you don't know how to use a knife then you shouldn't use one at all. You could hurt yourself or others if you aren't careful.
I think  you are wrong. I'm going to learn how to use it. It's courtesy to know how to hold a knife.
Well as long as you learn how to use it from somebody who knows how to use it. Otherwise you could cause yourself an injury or somebody else for using it when you don't know how.