Back reflections at low frequency?
One starts to worry about reflections at high frequency because the disturbance caused by such reflections (which are high frequency as well) will interfere with the signal. At low frequency, the first capacitor down the road will filter those disturbances out.

Reflections may happen regardless of the useful frequency of the signal. If the signal happens to have sharp edges (short rise / fall times), you may end up with twice the voltage at the end of the line.