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only if you get an annoying vibrating sound from the fans.. usually helps for the side panel fans that cause the panel to vibrate.. about the only place ive seen them help on..
I've used them on a couple builds and never noticed much difference. The problem is that it's very hard to compare without recording equipment or side-by-side comparison.
These fans are reasonably priced and move enough air to be effective, at least in the 1200 RPM category. If heat isn't a real issue you could drop to 800 RPM but I don't think the noise/airflow ratio is worth the trade-off at that point.
Your HDD WILL make noise if its firmly attached to anything metal. The rubber washers, etc. really don't make a difference so don't waste any money on those. A lot of new cases, especially Antec, use nice silicone grommets to insulate the HDD. These are very effective. Look at the HDD bay on an Antec P180 for example. With your current case you would want to consider a HDD enclosure (expensive), or HDD suspension (cheap & creative).
I know this was a long answer to a brief question, but I have a feeling that you're headed down the same road I was a couple years ago.
If you don't use the gaskets you keep your $5, but you might have to re-do your install if there too noisy or cause vibration problems. Guess it just depends on how much you value the $5.
I use them in areas where I am mounting to unsecured areas of the chassis that would be prone to vibrational problems.