Mostly personal prefrence, i run 30psi on the back, 25 on the front on my stock.
Taken from: www.TrashZen.com
"WHAT PRESSURE FOR THE TYRES?
It depends on the ground and the sharpness of the edges you climb on. The sharper the edge, the more inflated you want the tyres so that you don't get a double pinch flat when banging the back wheel on it (the unfamous snake bite, rim rails pinching the tube against an edge). For round blunt obstacles like wood logs, less pressure gives extra gripping because the tyre can litteraly grab the edges and shapes. less pressure also gives extra bouncing for landing, and is more comfortable. Fat tyres (2.5" or wider) are the best, and allow a lower pression than thin tyres, so more comfort and more grip."