Do I have to use a sense resistor? MC34063 Step-Up
The sense resistor is used to limit the peak current through the transistor switch and the output in the case of a short circuit on the output or the output inductor saturating. 

A sense resistor can be a PCB trace or a length or wire. For example, a 1.8 inch length of 40GA. copper wire will have a resistance of about 0.16 ohms at 25C. 

But, it does not have to be used.