It is not mandatory to register the WILL.
However WILL may be registered preferably.
A registered WILL may not be set aside easily at least on the counts of authenticity.
Any WILL can be contested.
If contested WILL lands up in probate court of pecuniary jurisdiction.
In presidential towns it is mandatory to probate the WILL.
You may inquire locally the charges to be incurred for probate (if such situation arises).