Is placing of thrust block necessary in HDPE piping system?

Specifically, piping systems where the piping material has sufficient strength to resist axial tension forces that can occur due to pressure x pipe inside cross sectional area, will have no benefit from thrust blocks. 

Piping with "push-on" type joints, with limited strength in tension may require thrust blocks.  Fusion welded HDPE, Solvent Welded PVC/ABS, metal welded piping, screwed piping, and geooved pipe couplings do not need thrust blocks. 

Bell and spigot type joints do require restraint form joint separation, typically in the form of cast concrete blocks in underground piping systems.

No, it is not necessary for HDPE piping systems, the fusion (electrofusion/butt fusion) jointing system of PE pipe/fittings are monolithic and fully restrained. 

Yes, thrust block necessary for HDPE piping. Especially of bends (90 degree to 45 degree) we need to provide Thrust block. Its only for High pressure piping.