Dynamat works a couple of ways. One is by just adding weight to a panel, thereby requiring more energy to vibrate it. Second, it is semi-viscous, vs a pure solid, which also helps suck up vibrational engery.

Covering an entire panel would maximize both of these, but smaller pieces can be used effectively, as well. If you put a strip of dynamat across a trunk panel, for example, it will keep vibrations from running across it...kind of like an island keeping the ocean waves from going through it. Obviously, the more you can put on, the better, but I don't think its linear...ie, twice as much dynamat does twice as much good.

The roof of my caprice only has three strips of dynamat on it...I think they are like 15x30, or something, and it doesn't vibrate much.

Now, if I'm doing a door skin, then my goal is not only vibration dampening, but rear wave blocking. I'll try and cover all of the factory access holes, keeping the rear wave from coming around and cancelling the front wave. Here, I'll probably end up covering the hole skin.

Short answer...some is better than none, and might not be any worse than lots.

Toby