This can help if you have vibration noises but a lot of the noise comes from the components themselves. For instance in a case that had a nasty fall once I will sometimes get a vibration that stops when I tap the case, but it also has quite a bit of noise from the fans pushing air through the radiator. So it just depends on what kind of noise you are trying to suppress as to how you should go about it.

I suggest using slient cooling components instead, such as slient fans and cpu cooler. First, track down the source of the main noise origin. ie, CPU cooler running on high speeds. Secondly, tune down the noise output of the source by reducing speed, replacing it, etc. Depends what you are looking for, you might have tradeoff in cooling & overclocking. Also you might have to replace step 1-2. But sound dampening material is used last just for "icing on the cake", you have to fix the source first. so this is my procedure of sliencing a rig:

1. Fans, Coolers, HDD, PSU (noise sources) replace with slient replacements.
2. Case (some tends to add vibration noise,etc)
3. anti-vibration pads/screws
4. Sound dampening

Usually I go as far as number 2 as many fans I purchase have anti-vibration features.

Also, note that different fan bearings will generate different amount of noise, usually fans comes with cases are sleeve bearings and will generate more noise as they wear out.

coming from a very picky person about noise and only favour slient computing.