One hears this about bicycle tires. However the equation for sliding friction has no term for surface area. IOW it doesn't really matter. Be that as it may, touring by motorbike through Europe, Africa, and the USA, I found that I got best performance in every weather by adhering to the manufacturer's recommended tire pressures. Not everyone agrees with that.

This website:
http://www.bikerhiway.com/motorcycle...er_riding.html
says, "Additionally, a slight increase in tire pressure will improve wet-weather traction."
This website:
http://www.netrider.net.au/articles/?page=wetriding
says similarly, " It is a myth that lowering tyre pressure helps the tyres to heat up and give an increased contact patch."

I get best performance in the wet with the same tire pressures I use in the dry. The reason is that the most important thing is that the tire not deform. Tires are made to corner on that part of the tread that the manufacturer expects to be in contact with the road. By lowering tire pressure, the shape of the cornering tire is altered out of spec.