It's on your understanding with builder many experts think that there is no need of registry because section 54 says that you should purchase the flat and the purchase is considered as complete when the payment is made and you are having the complete possession of the flat. Many experts think that purchase deed is not necessary for proving the possession of flat.
However as far as my experience goes I have seen income tax officer ask for purchase deed whenever there is any query relating to a property so I would suggest you to register your purchase rather than taking a risk.
Hope you find the information helpful if you do please rate it 5 and provide your valuable feedback for my improvement.
Thank you