I doubt this applies to feathering props most of the time. With fixed props the prop is applying force to the transmission
when in gear
and possibly causing the dog clutch
to move opposite the direction that locks it up which will cause damage to the dog or the gear. The only time this is an issue with the feathering prop is if you pick up some debris on the prop and the prop gets knocked out of the feathered position. I once picked up a piece of rope
which caused my autostream feathering prop to start rotating. It kept rotating until I got to the anchorage and could dive to get it off. Since I was in neutral it was just an inconvenience with no damage. Had I been in gear it could have rotated the wrong direction for that gear. Play it safe and keep it in neutral.