Never Use a Knife If You Don't Have To

This is key. Unless your opponent tries to use a knife on you, and you disarm him, and he's somehow able to get another knife, so the two of you can fight knife-to-knife, NEVER, and I mean, NEVER, initiate a knife fight against a knife-less opponent.

Why?

Because I said so!

I am just being silly. Of course, you can use a knife...if the above conditions are met!