For NAND flash, must spare area always be the last 64 bytes or 128 bytes of each page?
No. The spare area is meant to store ECC data and other metadata associated with the data stored in the main area. Though there are 64 or 128 Bytes of spare area in Cypress NAND flash devices, the spare area can be located anywhere in a page. Two options shown below are most commonly used, but many other options are possible.
Option I: Each main data area is followed by a spare area
Option II: All the main data areas precede all of the spare areas