I use the DALLAS DS1220Y. Nice little thing that directly replaces 2kByte SRAM. No problems at turn off.

BUT:
Sometimes strange things happen and these NVRAMs are not visible to the uC. If I pull the RAM out, drink a cup of coffee and put it back into the socket its working again. Because this effect is board-independant I lay the blame on DALLAS.
Still got no idea what's wrong. And if I'm waiting for this failure (to check the RAM pins with an analyser) everything is working. Murphy is everywhere...

Olaf