Big Bang, an useless skill?

I know BB is primarily used by F/P AMs as an additional source of DoT:
1) Is there a difference in the DoT when casting it uncharged vs. fully charged?
2) Is it better in terms of DPS to cast it uncharged or fully charged? (i.e., When bossing, should we just tap the key so that the DoT is applied, or is it worth the time to charge it up all the way since we have to cast it anyway?)