how does one determine what to set the stepper motors step switches to?
I'm assuming you are referring to micro-stepping.
According to the Mach3 manual, microstepping is mainly used to reduce mechanical noise and possible resonance problem. Hence there is no specific answer to your question. If you go too high in microstepping then there will be an upper RPM achieveable due to the maximum kernel frequency of say Mach3, or EMC2.
As an example, my Gecko G540 is fixed at 10 microstep setting.