Air pressure has nothing to do with valving, so no, you don't understand. Air pressure resists the oil foaming and cavitating and so helps the shock valving do what's it's supposed to do when the shock is working hard and gets hot. Once you get a shock really dialed in for go fast stuff you might play with the pressure a little, but for rock crawling and trail running you don't need very much pressure. Excessive pressure will add a little to the spring rate, like an air shock, which you probably don't want for crawling. Most people will never feel the difference between running 125 psi or 200 psi.
Really, we need to know what shocks you're talking about. If they're air shocks, none of this applies. If they're smooth body shocks and not coilovers, there is no spring rate. If you have coilovers the spring rate is with the springs, the air pressure is just to keep the shock oil from foaming when it gets real hot.