I'm not sure if/how you can do that easily, but you could use something like the
DeepSleep widget on your dashboard, so that the normal ways of putting the computer to sleep cause it to be asleep (safesleep will still make a ram image), but the widget will actually trigger a hibernation. That's fairly close to what you want.
I don't think you can dynamically turn off going into safe sleep. There's an important distinction in terminology....
(Normal) Sleep - no RAM image is made, if the computer shuts down, then you lose everything, but the computer goes to SLEEP.
Safe Sleep - RAM image is made, but the computer still goes to sleep rather than hibernating, and stays asleep unless it loses power.
Deep Sleep or Hibernate - instead of keeping the RAM image around just in case, the computer actually shuts down right away, and then when it is turned back on, it boots the RAM image instead of having to boot up normally (which is a lot faster).
I'm pretty sure turning the creation of the RAM image on and off requires a reboot. So you can't choose between normal and safe sleep dynamically. You *can* choose between safe sleep and "deep" sleep dynamically (i.e. as above, using that widget for deep sleep).
Does that make sense?