You need to control the step rate acceleration when the step torque is less than the torque needed to overcome:
- the driven inertia i.e. resistance to changing speed (not momentum!)
- frictional losses
- electrical and magnetic losses
- any work load.
The step torque depends primarily on the motor specifications and current flowing, the torque and will decrease with speed for various reasons so the acceleration limit at higher speeds will be lower to the point where it will not be possible to accelerate at all.
You can calculate the acceleration limits at different RPM by computer modelling the system, or you can find out empirically.
If the rate of change (acceleration or decelleration) of the stepping frequency is too great then angular steps will get missed and the stepper motor will not turn through the expected angle.