I use cover on turn 4 BCB at the start of Phase 4. Our WHM opens up with a big heal 99% of the time, and does it on purpose so it's easy for me to pick up the adds so they don't run all over the place. I have 2 cover macros,

/ca "Cover" <p3>
/ca "Cover" <p4>

When we go to a premade I set the healers as part slot 3 and 4 with the MT as Slot 1 and the OT as Slot 2.

So after the dreadnaught dies in phase 3, I press cover macro 1 to cover the whm (party slot 3) so I eat all their dmg while the MT gets the 2nd rook and I pick up the other rook and the 4 spiders.

I also use cover on turn 2 once in a blue moon if the MT or I screw up our taunts and he/she has to many stacks so I cover them while I wait for mine to drop to taunt. I also cover the melee (we have 1) from time to time just because I can. E.g. if the MT has the ADS I can cover the melee so they don't take much aoe dmg.