Flat areas. Contours and stamped areas don't vibrate like flat areas do. So if you're on a budget, you'd be best served by putting any vibration damper such as Dynamat on the areas that are flat.
If you want sound control such as cutting down on road noise, you shouldn't be using Dynamat anyway. Dynamat is for vibration control or panel dampening. This will make a door or floor feel more solid when you close the door or knock on the panel. It'll help with road noise from the simple idea that a thicker piece or solid material will be harder to hear through. To cut down on road noises and make it quiet on the inside, go with a barrier. You can use jute backing which is what's below the carpeting in a house but it absorbs water like a sponge. It's cheap and works great though. There are other materials on the market that are better designed for the automotive environment. Closed cell foam is supposedly the new hotness.
Check out this company. They make better products that Dynamat.
http://cascadeaudio.com/car_noise_control/index.htm
Dynamat makes good stuff but they advertise better than anyone else. They aren't the best by any means.