Let's cut right to the chase - if you are under attack do what ever you can do to get away. That being said, if you can avoid using your fists/knuckles you are probably going to be better off.
I can hear the cries now, "But David punching is where it's at! And besides, your logo is a fist!" I agree. Punching is fun to practice especially with gloves on when hitting a bag or the focus mitts. But in a real street encounter, I avoid hitting with my fists/knuckles. I have seen too many people miss a real target and end up hitting someone's skull, a wall, or even the ground. The end result is that you can end up blowing out your hands.
TIP - hit with the heel of your palm &/or the edge of your hand. These are some of the densest parts of your body and they can withstand lots of impact. And guess what? You can still throw a jab-cross-hook combo with the palms! Don't know how? Let me teach you!