When I'm off road, I run around 18 f/r, and I haven't tried running any lower, but I'm sure I could get away with it.
When I'm on the street, I run street pressures in my tires. 33f/35r or somewhere in that neighborhood. This gives me better tire life by running cooler, and since I'm running Michelin Anakees, they're rated for pressures higher than this.
The factory "setting" is just a compromise between street and dirt pressures, and in reality, sucks for both conditions.
I run street pressures all the time, and when I get to something that I need lower pressure, I air the tires down. When I'm done dirt riding, I just top them back up at the next filling station.