Should I twist DC power and ground wires from a power supply?
Twisting wires reduces the magnetic loop area of the wires, this has two implications:

Reduced susceptibility to noise from magnetic fields, with twisted wires an a smaller magnetic loop area, external magnetic fields will induce less current in the loop made by the wires than straight cables.
Reduced magnetic radiation from loads that are switching. A magnetic loop is an antenna, switching loads and changing currents cause the antenna to radiate magnetic radiation, which can cause noise in other devices.

Generally its a good idea to twist the wires and reduce the radiation and susceptibility to noise.
I can't think of any reason why you wouldn't do this, on all of my prototypes/products I make sure the wires are twisted.

In the image above the magnetic loop area of a straight cable is large, a twisted cable the loop area is similar, but the adjacent loops magnetic contribution cancel each other out.