What values are the Atmel MCUs EEPROMs preloaded with? I have checked with two datasheets: one for some ATMegas and one for some ATTinys MCUs and could not find what the EEPROM.get() call would read back if the EEPROM was empty (or contained default factory values). I have written a program in order to find out and it looks like on Arduino Uno the EEPROM bytes are all FF by default. 

Is this the case with every Arduino board or just some of them ? Or perhaps we can not assume that all the MCUs of a certain type (such as ATMega328) will have this factory preloaded value of FF for the EEPROM bytes ?

edit

Following the fuses - thanks @Chris Stratton - I was able to find this in the datasheet:


  Skip writing the data value 0xFF, that is the contents of the entire
  EEPROM (unless the EESAVE Fuse is programmed) and Flash after a Chip
  Erase.


Seems like we could get the factory pre-loaded value by not programming the EESAVE fuse, which is what the Arduino Uno does by default, and just reading them.


  uno.bootloader.low_fuses=0xFF
  uno.bootloader.high_fuses=0xDE
  uno.bootloader.extendedfuses=0xFD


However, having a way to set them to a known value is fine too for my needs, so I shall just write that fuse and havea  guaranteed 0xFF all over the EEPROM.
They're all set to 0xFF by default.