From what I understand, the sound deadening material, is more of a damper, and not insulation (absorption) against sound. So, even a small patch on a large flat panel will dampen or reduce the resonant frequency of that panel, hence quieter. Also, will dampen any reflected sound waves.
The secondary affect is absorption, resulting in a reduction in noise. I could not find any numbers about the amount of absorption, even on the dynamat site...
So, using this facts, you don't need to "seal" the entire surface, and as Nate suggested, it's fine and best to leave some gap from certain areas.
And if overall noise is the goal, the underhood seems to be the biggest offset. (Surprise!)
Reference: Putting Dynamat to the Test in a Ford F-150
It's going to be interesting, and I want to do this in steps and see the affects. The one recommended area, that I will not be doing, is the headliner. Sometime in the future, when I get that redone, I will install mat there. but not before.
Would be interested in any ideas. (and counter-ideas.)