It depends on:

1. Your weight;
2. Tire size and tread;
3. Riding surfaces.

What you want is 'as low a pressure as you can go with the tires WALLOWING in a corner'.

I've run DMR Redshift 2.25's as low as 40psi (they feel good to me between 40-47, and I weigh 235), and they stick like crazy, even in SNOW. But 40-45 psi in Maxxis Ridgeline 2.10's will wallow like a drunken, three-legged pig. (Oh, and my spiked winter commuters max out at 40 -- they do fine, too....)