In ionic solids such as NaCl, do the cations fill interstital holes in the cation's cubic close pack structure?
The relative radii of the ions, and the stoichiometric ratio of the ions needs to be considered.  

For NaCl, the radius of Na+ is 0.695 that of Cl-. 

Na+ ions occupy octahedral holes in the cubic close packed lattice of Cl-.

If the radii are more equal (ratio greater than 0.732) then each type of ion will form a simple cubic lattice, with each ion of one lattice filing a cubic hole of the other lattice. For example, CsCl.

If the radii are very different (ratio less than 0.414) the smaller ions will fill tetrahedral holes in the cubic close packed lattice of the larger ions. For example ZnS.

See http://depts.washington.edu/chemcrs/bulkdisk/chem162A_sum06/handout_Lecture_11.pdf for more information.