I would always teach to keep the hand on the throttle. During any phase of flight if the spare hand isn't doing anything else it should be on a throttle.
I once flew with a very experienced RAF instructor, and he commented that above 1000' my hand wasn't on the throttle. He said that during his training, if he wasn't holding the throttle for an extended period and his hand was just idle, the instructor would simply cut the throttle. This would keep happening until he had built the reflex to keep the hand on the throttle.
I have also had the case in a 172 where after take off, having forgotten to tighten the throttle friction, the throttle started to come out. Rather disconcerting to have a sudden loss of power shortly after take off, and that was the only lesson I needed to keep my hand on the throttle!