Should one complete registration before flat completion?

If the buyer books an under-construction flat and gets loan from a bank to purchase it, is it advisable for the buyer to complete the registration process when the flat is almost completed (though not actually completed and electricity connection is yet to get along with completion and occupation certificates)? The reason why the builders are keen to complete the registration process is not clear, perhaps they need money to continue their another project etc (in reality they are advising buyers to do it before some long period considered inauspicious by Hindus starts!!!) Another issue may be that already due to delay, they had to make supplementary agreement to extend date of completion to satisfy the bank sanctioning loan to the buyer. Whatever be the reason, my doubt is should one complete registration before actual completion of the apartment? What if after registration the builders delays the project indefinitely? Will the buyer be able to do anything? Please advise.