If you want to extract the most out of your batteries, wouldn't you use a boosting reg so can extract every last bit? I think for most projects, the uC uses far less power than the auxiliary components around it, like a LED. If you boost the voltage you can use higher logic levels. There are many examples boosting a "dead" 0.5V AA battery to 3V at a lower current.

If you use a buck regulator, you could use starting voltages beyond the chip's max voltage limit and burn the power past a typical dead cell's voltage levels -- start with a 12V battery using 1.8V logic and burn the battery down to 2V -- probably around 98% used.

I think you'll find that most projects out there use a voltage regulator because you can use up "more" of the battery this way.