FWIW, this is what I do:
In the circuit, I have my hand on the throttle all the time (except when moving flaps, carby heat and such).
When out in the training area, once established in the intial departure climb (say 2000ft+) I take my hand off the throttle and rest it on my leg (not exactly far away) and when in the training area have my hand on either the throttle or my leg depending on what I'm doing (i.e. in cruise, hand not on throttle, doing things, hand on throttle). Unfortunately for me, my arm isn't long enough to rest my elbow on the window and have my hand on the throttle, otherwise I'd do that (although if anyone has a way for short people to manage that, I'm all ears).
I'm not up to navs yet but I imagine I'd be doing the same as in the training area.