Should one use Paper White C0 M0 Y0 K0 or 0% of a spot color when doing a full white knock out of said spot color? Also, if the white knockout spans more than one spot color, does the same apply?
If the production method is spot printing or process +spot, it doesn't really matter if the knockout is 0% of a spot ink* or CMYK White. Any object that is white in appearance and is NOT set to overprint should do the trick, with a few caveats. Be aware that RGB white isn't always translated to CMYK white by all RIPs. Also be aware that black tends to overprint by default so 0% black may vanish when plates are printed. That is because when a 0% object is set to overprint, it prints nothing and knocks out nothing.
*If the ink or objects that the ink is applied to have overprinting attributes turned on, then you need to either disable the overprinting attribute or switch to CMYK white.
Regardless of which method you use, always view the separations either by printing each separation individually or by previewing the layout separations preview mode. Separations preview mode is supported in InDesign and Illustrator (and probably other modern print-oriented layout software) and is a real time composite of all plates as they should appear when printed. Normal view and office printers cannot be depended upon for this.