@soix - if a cable is shielded, then it should only be grounded on one end, usually on the source side. If you ground both sides, then you are creating another full circuit and turning it into an antenna.
@billpete - I worked for company for 32 years that made speciality metal alloy wire, tubing and machined parts for the medical device market. I was the 5th employee of what is the world leader in the market. And I have drawn my share of both. (FYI, the process of making metal tubing and wire from larger diameters, is to "draw" it, not to extrude it. Extrusion is usually only done from the cast initial billet of metal by pushing it through a die. In drawing the metal is pulled through the die.)
In a nutshell, the reason for the directionally of electrical flow is due to what is called the grain structure of the metal. (Think of it like the cells in a leaf.) As the material is drawn smaller and smaller, the grains become elongated (smaller and thinner) and pointed in the direction of the drawing force. The surface of the material is also undergoing changes in stresses. And since electrons move along the surface of the metal, when they hit the grain boundries and jump to the next grain, it affects the electrical flow. Thus, the electrons will flow smoother in one direction better than the other.
@roadcykler - Not to start a war here, but it is physics not sentienticy, and grain configuration/stresses in the metal do affect electron flow. And that can be tested and measured. A classic measurement is to take a straight wire and measure the resistance. Now take that same wire and bend into a tight curve. The resistance will go up because you are stressing the metal where the electrons flow across.