If You step the motor the proper way, within its limits, no encoder is needed. An encoder could be seen as the seatbelts in the car. In case of accident it saves You. Drive without accidents and make the seatbelt is not needed.
An overloaded stepper stalls and doesn't move at all from the point of overload. It just makes noise. A DC motor will keep on fighting but steppers don't. They give up.