How to have my MacBook Pro safely hibernate when out of juice?
Newer versions of OS X support restoring state when you log in. This means that any applications that were open when you lost power, shut down the machine or rebooted would open back up to where they were previously.

Using this mechanism, there is no need to write the 'hibernation state' to the disk, which (a) uses a big lump of hard disk space, (b) takes time and (c) is risky - if there's not enough power to complete writing the data, you lose all of the state.

You can use the following Terminal command to configure this if you prefer:

sudo pmset hibernatemode 25

per the help at man pmset:


  hibernatemode = 25 (binary 0001 1001) is only settable via pmset. The
  system will store a copy of memory to persistent storage (the disk),
  and will remove power to memory. The system will restore from disk
  image. If you want "hibernation" - slower sleeps, slower wakes, and
  better battery life, you should use this setting.


...but please heed the following warning in the help (man pmset):


  We do not recommend modifying hibernation settings. Any changes you
  make are not supported. If you choose to do so anyway, we recommend
  using one of these three settings. For your sake and mine, please
  don't use anything other 0, 3, or 25.