Can a series resistor be used to limit the power consumption of a phototransistor?
Typical forward current is shown as 1 mA with 20 mA current on the LED side. 

So 5 mW consumed by the receiver.

And 24 mW by the source considering the ~1.2 V LED forward voltage. If you're just using a resistor to limit the LED current, then you'll be consuming 100 mW for the source.

Whatever power you save by reducing receiver current won't do much for your overall system budget. Also, it will reduce your sensitivity to objects at farther distances or with non-reflective surfaces.

A better way to save power, if you have a nice reflective target or only want to detect it very close, is just reduce the LED current. Then you save power on both sides.