The general knowledge is that dungeons wise, you don't ever need to use CDs for small pulls. However, you would want to pull more than that if you want to get by it faster, usually 2-3 groups of mobs at 1 go. Now, that's when you want to learn how to pop CDs appropriately if you want the best efficiency and pretty much less time killing mobs/bosses in dungeons.

It also depends quite heavily of what kind of DPS composition you are running with. Naturally DPS jobs that can AOE well is going to destroy a large pull a lot faster, unlike a few jobs that are restricted on resources to keep doing AOE (TP issue mostly)

As for tank busters, most of those hit like noodles if you compare those to the hardest raiding dungeons so they aren't as painful. Most of the harder bosses (trials/raid) will have their own patterns on tank busters. Most inexperienced tanks will outright fail because they blow a lot of their cooldowns and not planning them better for the next sets of tank busters.