Hibernate or off completely - not sleep, at least until Vista.
Sleep is still a "power on" state, requiring power to maintain current state in RAM, while Hibernate writes the state to disk (hiberfil.sys) and then powers off.
Vista allows Sleep to go to Hibernate when power runs low.
But in case something brings the lappie out of sleep when in transit - and still with sufficient juice - best not to risk it.
SD