Judging from the replies to the question there is no compelling technical reason for the pilot to use both hands for rotation (see Airbus sidestick). That leaves tradition and myth. Taking your hand off the throttles just in case you succumb to a reflex action of jerking them back if you get a fright, suggests a myth. Like the reflex action of slightly lifting your bum off the seat when you know the other pilot is about to bash the Boeing on the bitumen. "Bit late on the flare, Bloggs...."
Different I believe if you are being shot off an aircraft carrier where the enormous power of acceleration can cause you to pull back the throttle(s) - hence hand is kept at the back of the throttles and not on them.
That leaves tradition. And tradition is just another name for someone's personal opinion years back.
Verdict: One handed rotation is perfectly safe - it's done every day on light twins and singles.