If you want to do it, go to partsexpress.com and buy the super cheap stuff ($20 for 4'x5' sheet, i think) compared to whatever price that dynamat sells for the equivalent amount. All it is doing is adding extra material to the case, which 1. weighs it down causing less vibrations and 2. forces the sound to go through more material.
If it does anything noticeable, you will realize how loud your psu is. I did it because i had some left from my truck and it helps a tiny bit, but i also added a layer of foam to get the higher frequencies. All i hear is my psu, but that is on a controller so i can adjust it to my preference.