The powerizer spec is written for standard discharge down to 2.75v (multiple tests use that criterion, including capacity and total number of charge cycles).
I've never found a reference to minimum safe working voltage, which will be a value less than that.
I run them until I get either low output (both LEDs lit) or end of the day. I meter once in a while, at end of use and fresh off charger, haven't ever seen a really odd voltage.