Also, for any multi target fight, maximum number of bombs should be placed, this includes VnT, Halfus, Ascend council, and Cho'gall. Heroic maloriak black swills should also, obviously, have bombs rolling on them the entire time, but I'm not sure how far your guild is progressing into content so I dunno if you have to worry about heroics yet.
Fights to not multi-dot/aoe:
-- Nefarian, VERY bad idea to try to dot the adds, you will more than likely be scolded by your tank, If you can, double bomb ony/nef, and maybe keep a bomb on your chromatic add during p2
-- Maloriak: Until the aoe phase, the adds take significantly less damage, making the GCDs to put bomb on the adds a large waste of dps
-- Omnitron Defense Council: Unless the shields are down, multidotting the golems can easily cause unnecessary raid damage. Once the shields have deactivated, multibombing wins.