I would use one on any engine anytime the boat is towed
Certainly anything bigger that say a 10 horse, they are super easy to use and can save some very expensive damage, seems worth the 3 seconds it takes to put one on.
As a general rule of thumb. The larger the boat the larger the motor, the smaller the boat the smaller the motor. The size and strength of the transom matches the boat size and both can be stressed if not supported properly with a transom saver.
If the boat cannot be towed with the engine full down then you need a transom saver. If there is at least 12 inches clearance between the skeg and the ground you don't. Just don't back into any rocks, drive off high curbs, etc.
