No you don't want to switch off the donkey, particularly in winter as the heater stops working!!
Like you WWW I have done lots of eng off in the motor glider. Thats okay as we typically lose 200FPM. In the Cherry Tree its more like 1000FPM. Much less time to restart the eng in case of b*lls ups!
I was certainly never taught to switch the engine off in the air during my AFI.
Did switch the right engine off in the Seneca during my twin training. We almost maintained straight and level flight!