Keeping in mind tire size and structure as well as plow weight and the type of snow will all play a factor, generally around 6-8psi. You should also set the front shocks preload to MAX and add some weight to the back rack to counter balance the weight of the plow as well as equalize traction. I used to ratchet-strap two solid 8x8x16 concrete blocks to my back rack when plowing. Was a great help.
It all depends on how wet and heavy the snow is you are trying to push and how deep. I have always just put about 2-3 extra psi in the front tires and never worried about adjusting the preload or weighing down the back racks. But I live in Utah and most of our snow is pretty light and not super wet and heavy.