From the location allocation desktop help on this topic
Demand points have an ImpedanceParameter property, which, if set, overrides the Impedance Parameter property of the analysis layer. You might determine that the impedance parameter should be different for urban and rural residents. You can model this by setting the impedance transformation for the analysis layer to match that of rural residents and setting the impedance transformation for the demand points in urban areas to match that of urbanites.
which suggests that you can apply a transformation if you suspect that there is a difference in the behaviour of different areas and/or groups. This of course would have to be justified based upon some other analyses, so unless you know otherwise, then you would have to assume that demand etc is equal across the landscape/individuals and no transformation would be needed.
