> It might depend on the type of battery you are using.  If you're using
> a battery technology with a steady downward ramp in output voltage (ie
> carbon/zinc), there may be significant engery remaining in the battery
> AFTER the voltage drops below that which will allow a linear regulator
> to operate your equipment.  Some of the switchers are starting to publish
> boost/buck designs that provide correct output voltage with input ranges
> from below to above the target voltage.  After all, you're not really
> optimizing for current, you're optimizing for batery life...

If you use such a best, however, be very careful to ensure that your system
behaves reasonably (i.e. shuts down) when the battery voltage gets too low.
Without attention to this seemingly-minor detail, you may discover that as
the battery goes down the current goes up to the point that either the bat-
tery or the regulator explodes.  Unlikely with low-power applications, but
something that should be checked for nonethtless (if there are two or more
cells in series and one of them is completely dead, letting the others drive
excessive current through it is a good way of promoting a cell rupture).