i guess there's a concrete way to find out ... hook the processor up to a battery and see what happens
I use a 12f683 with brown out reset in a battery powered app, and the chip runs routinely below 3v. When the bats are fresh, I maybe see 3.2v, and before long they are down to 2.5v. The circuit actually works well to just under 2.0v, but once the voltage goes below 1.9v the pic starts resetting (BOD kicks in). By checking the flags to find what the reset cause was, if I find it was a BOR, the program will sit in a 'standby' mode and refuse to turn on the rest of the circuit (which is a switch mode boost converter powering ~25v of leds)