Just try it and see. Well worth having on if your computer is still stable as when just surfing you will be using 200W less power compared to a heavily overclocked q6600 plus you can have a quieter system to boot.
That equates to about 50p a day or about £180 per year. Of course, you won't be running at the lower speed 24/7.
And as said, I can run mine while surfing with my watercool setup running passively so total silence.
However, I could never get my system stable with old overclocked rig with them on but my new rig is so best to just try. If it works, leave it on