I have also seen issues with the 12 gauge generic speaker wire from Home Depot. I don't know if it was a bad batch, but I saw oxidation of the copper under the insulation, though not as bad as the example in the page Mac linked to. It took a few months for it to go from looking good to looking bad. I've used the Parts Express wire with no trouble for awhile now. It's about the same price as the Home Depot wire.

Generally speaking, 14 gauge wire is more than adequate for speaker runs less than 30-40' when the load is at least 4 ohms. While 12 gauge is overkill, if the price isn't bad, it doesn't hurt to use it.