Unused bits on a DDR3 chip
Unless you just ground them with 1K resistors you can add more code to insert frame or folder or page markers on the fly. While not needed in most cases, it can help identify/verify burst-mode segments, etc. I would avoid inserting data that has no relationship to data at the same addresses.

The wrong thing to do would be to let the pins float. As outputs it would be no issue but as inputs they need a 1K resistor to ground. This grounds them if they are inputs and is a mild load (1.35mA per pin) if they become outputs set to '1'.