Whenever You need to, if You need to. Guarding the thrust levers in any phase of flight other than approach will most likely do more harm than good as there is nothing You need to do with these thrust levers except if you experience wind shear. All the other possible scenarios of thrust levers handling are pretty much covered by non normal procedures.
I doubt the romanian A310 crew had much time to contemplate non-normal checklists when they flipped over due to a throttle jam. Enlighten me as to what harm my hands do on the thrust levers after gear is selected up.