I wouldn't ever use 16 or 18 gauge speaker wire. It's probably too thin for modern low impedance speakers, and 14 gauge or larger stranded wire is cheap from so many sources, including your local Home Depot, there's no reason to skimp. As Adam says, there's also no reason to pay premium prices for mere wire. Termination quality can vary, however, including the mechanical quality of the spades or banana plugs, and the workmanship of the termination operation itself also varies. Some folks like to run unterminated speaker wires, but I don't recommend it. Stray copper strands can cause shorting problems.