I think you have to qualify both the type of chuck and the mechanical properties of the workpiece before making any kind of blanket statement of this sort.
For the workpiece, if you expect a degree of plastic deformation (creep) under the jaw load from which the part will not recover, then unchuck it.
If you got a high-force hydraulic production chuck, it's probably best to unclamp in case the system starts to leak, and to reduce stress on the chuck's internal components (if you didnt stress them by clamping things, they would last forever).
If it's a high precision air chuck such as made by MicroCentric or Northfield, where the sliding components are fitted closely with minimal operating clearance, leaving it clamped can cause the jaws bind as lubricant is forced out of the sliding surfaces, thus making unclamping the following morning a bit of a sticky proposition.
Finallly, if we're talking an average key-operated scroll chuck and hunk of solid material, it probably doesnt matter one way or the other. I fI had to continue working on the piece the next day, I'd leave the setup as is to avoid the inevitable inaccuracy and waste of time caused by trying to reclamp.
