Selfin
Depends what you are doing - NACA Report 837 points out that you must use dynamic pressure for reducing flight test (and wind tunnel data) into coefficient form and when working with structural loads (and aerodynamic forces generally). Impact pressure (total pressure) is used by the engine designers as their non-dimensionalising parameter. Pilots generally couldn't care less about either when they are actually flying - they must see and fly to CAS, because that is all they can be given.
It would be perfectly possible with today's technology to insert altitude compensation into the ASI's laws so that the indicated CAS was more nearly equal to EAS at all altitudes. That would still meet the airworthiness requirements that the instrument shall indicate the correct speed(s) at ISA sea level, and the change would be transparent to pilots because they would still fly to CAS limits. The only difference would be the underlying EAS value at any given CAS.