Your question is answered in the OP. Whichever case it may be, general or specific (such as Valks), use your judgment, seriously.
Originally Posted by neomasterc
Think of aoe as more of a priority. Spread diseases>DnD>BB.
What about Desolation? Sometimes I ignore it and spam BB, but I wonder if it's worth it to refresh it while doing AoE on Val'kyrs for example. Usually I try to BS once just before the Val'kyrs come down.