http://www.wowace.com/wiki/DrDamage
I use this add-on. It gives me a damage per second of cast time number on all my dps spells. that way I always know what is best to cast. I find that with my stats corruption and immolation are still higher dps than shadowbolt, so I keep them up. I think this will be true of most destruction builds.
I also suggest people check out the wowace add-ons. Very convenient to have a single tool to update all your add-ons.