I have marked my tires, to make sure I wasn't spinning them at 0.. I wasn't, but yes, you DO need to be aware of that possibility.
You really don't need anymore than 20-23 on the street in the first place.. bet your ride will be a LOT better with less air..
Don't forget that airing down also lowers your final drive ratio too, not a ton, but every little bit does help, and it also decreases the resistance that rocks and obstacles in your path put up to your tires, it allows the tire to conform to say, a rock, instead of having to "climb" it, it just conforms to it and rolls right over.
There are several very real benefits to driving off road with very low air pressure.
Anymore, I have softened the sidewalls such that I can't run 0 anymore.. I have to have 1 or 2.. at least..
but yeah, on stiff tires, like my TSL LTB's, you can run them WAY LOW without too much worry, of course, I would LOVE to have a set of Staun internal bead locks.. that would rule, but they cost more than my tires... lol..
It all depends on the tire, you want to run them where they squat real good, but not enough to fold the sidewalls, or bottom out.. each tire is a little different. I had Buckshot mudders in 235x75 and I could run them down to 8 or 9 at the lowest, but they tended to do best at about 10-12.
These LTB's are nearly perfect at 5 psi, but with a spool, I can't run them on the street, they get too much traction, and I am afraid I will twist my axles at the splines.
Give it a try, I was truly amazed at the difference it made on my Tracker, even more than it made on my full size trucks.
The contact patch increases a WHOLE LOT.. I am not sure how much, but it seems like at LEAST twice as much.. in length mostly, but a little width too.
Here is an example: Taken from an EXCELLENT article here: (good read)
http://www.4x4now.com/sfjun96.htm