I think the idea is to trim until you reduce the control force. If you just use the trim control there is no control feedback for whether it was put to the desired position and it takes a bit of time for the aircraft to pitch and settle. That's not to say you couldn't learn how much to use in a given situation on a particular type of aircraft.
One of my PPL instructors taught me that in a C152 if your fan stops, 7 back trims will roughly give you 65 kts from a 90 kt/ 2300rpm cruise. Generally it works too.
With regards overstress, I imagine the cotrol mechanism will have some bearing on that. If you have a geared trim wheel it will be easier to make finer trim adjustments than with a lever and harder to make abrupt adjustments.