It's best to preemptively Cower rather than to pull aggro and then Cower. If you pull aggro and the Cower is resisted or isn't enough to drop you below the top aggro target then you're as good as dead thanks to the 10 second cooldown. For example, tank builds 2000 threat on skull target. Meanwhile, as DPS kitty, you dealt enough damage to skull target to acquire 1500 threat, 700 threat short of the 110% melee threat cap. Using Cower (Rank 5) at this point drops your threat to 330 threat, preemptively keeping you from the top of the threat table. This is better than using Cower upon reaching 2200 threat because at this point the tank has to overcome your 110% threat cap, 2420 threat, in order to re-acquire aggro from you without using taunt.