Spend about $10 (during installation) and extend the life of your water heater by preventing galvanic corrosion. Volta invented the battery (dry cell) by putting an acid paste between two different metals. So when you join copper and steal pipes together and run water through them you will get a bit of electric current that deteriorates the pipes and takes calcium salts out of solution. Galvani is the guy who connected Volta's battery to frogs' legs and made them twitch.
The dielectric effect can also be seen in a series of pinhole leaks at the bottom of copper pipes:
Iron particles from a rusty water heater lay along the bottom and corrode the pipe all along its length...yet another argument for replacing water heaters every 15 years! Similar problems near joints can be caused by excess flux inside during soldering and turbulence caused by burrs not smoothed off before soldering.
I've recently seen these leaks around the water main entrance to the house. I guess there would be a lot of turbulence at this 90 degree angle. Some sources say that any minerals in the water would scour the inside of the pipe. There are also requirements that any joints be a certain distance apart, to reduce turbulence.