Not true since 5.0. Provoke generates a bit of extra enmity on use now. Iirc it's about a 2000 potency worth, more than enough that your co-tank shouldn't steal back unless they all-out IR or you drop stance.
Tank stance is 10x the emnity. A 2k tank won't lose aggro to a 19k BLM. (However a dps CAN steal mobs in dungeons with heavy burst from a tank who hits a mob once while running). If you're a tank and struggling to keep aggro from a co-tank without stance, they're definitely voking. If you're stealing aggro from a tank as dps, they either have no stance or simply aren't using any abilities.
Heal actions have reduced emnity since 5.0. It's as low as 0.35 for WhM/Ast. Paired with tanks 10x, Regen/Medica II will almost never rip agro if your tank has hit the boss/mob.
And that's todays lesson on 5.0 emnity.