I only do Principal and/or Supporting role work, should I still create a profile with POP?
Absolutely!
Some of the casting directors and companies using POP will only cast Background Actors, but some will be asked to provide stand-ins, doubles, supporting roles and principals.
The inquiry will define what type of work you are being offered.
You can always choose which roles you accept or decline, and this will have no impact on your artist profile whatsoever.