Yeah, the cover up defense only really works if you are able to step to the side of your opponent while being hit or hit back instantly. If you are taking multiple shots to a cover, the second or third one is going to mess you up.
You are basically turning yourself into a punching bag. At higher levels, no one is even going to be impeded by a static guard anyway.
That said, for the one shot you should take on a cover before you step, hit or clinch, you want tight fists, pushed against your crown, with shoulders up and chin down so that a fist can neither fit through to your ear, make you hit yourself, or find your nose.
I only cover when I lose track of my opponent and don't know what his next shot is. I am careful to always follow my intuition and immediately clinch and hide my head or step off at an angle, find him, and fire off a straight right away.