Wet = less pressure.
Rocky/hard = higher pressure.
The compromises aren't hard to work out:
Less pressure means less stability, increased chance of ripping out valves (Rossco will be delighted when I inform him of the new-fangled invention called a 'rim-lock'...
), more traction on soft/uneven surfaces, better clearing of the tread in mud, greater chance of a pinch-flat.
For racing, I start with around 13~14psi F&R and then adjust to suit. I never go below 10psi, and never above 18psi...
Trail-riding, I tend to add a bit more pressure because I feel that the small-but-definite reduction in traction is worth the extra puncture resistance.
I usually run more pressure on the short travel bikes for much the same reason. Maybe I should spend some money on suspension...