I know this has been covered somewhere but I didn't see it.
On a typical affliction rotation you put up corruption on your first rotation thereby making me believe that your buffs and the bosses debuffs have no bearing on when it should be applied. So casting corruption while the +700 spell power trinket proc is up does not give your corruption extra damage the whole fight, likewise you don't have to wait for CoE to be on a mob before you cast corruption as it will benefit from it anyway. I believe this to be correct, however I haven't played affliction since 3.1 and looking around there seems to be a lot of contradictory info.
One thing I did see that I wanted to check is that, while a + spell power bonus won't increase your damage, applying corruption while standing in a boss buff that increases your damage, such as a rune of power on Hodir, will increase corruption for the rest of it's duration. Just wondering if anyone has the truth about all this crap cause every website seems to say something slightly different.