Hi,
PTC is self-regulating. In worst case both regulation systems are fighting each other causing oscillations.
In best case (for stability) one regulation system dominates the other...making the other regulation system useless.
If the PID regulation system is stable (without PTC) then the use of PTC may useful for safety reasons: to avoid overheat (fire) in case of erroneous operation. (Failure in PID regulation, temperature sensor failure). But then during normal operation the PTC should behave neutral - not to influence the PID system.
Klaus