If I ever pull aggro off the tank, I run over next to them and target something else so they can grab it back easier. Feels like that's the right thing to do as I'm not keen on lowering my DPS output, if the tank is still learning I'll throw up Quelling Strikes as combat initiates but that's not always available. If a good tank does lose aggro to you they'll have it back in no time if you help them out a little.