In reality, I'm sure 14g would be just fine. I just prefer to go a step higher on these things just to be safe. When I bought my Swans a few years ago I bought some Royal brand 14g speaker wire from the automotive dept at Walmart. It was $9.99 for a 50" roll at the time. Seemed like very good wire. It used a lot of very thin strands of wire and a thick but soft jacket making it very flexable. But when I finally ran the wire under the house I went with Monoprice 12g CL-2.
Also, for those of you who are not familiar with monoprices CL-2 rated wire, it is wound a little tighter than most wire and the jacket is made from harder rubber/plastic than what you might be used to. This makes the speaker wire itself a bit smaller than other 12g wire but it is also much less flexible than you might have hoped for. I'd also bet that the outer white jacket around the two wires is the same one used for all gauges so the actual total thickness of the wire as a whole isn't much different regardless of the gauge. I've never compared their 14g directly with the 12g but I bet there isn't much difference in the size of the outer jacket.
It also looks like their website is down until Tuesday!

Maintenance

And that link Al gave to the Secrets dual 14g wire was interesting. I remember hearing something like that before but didn't give it much thought. Even though most of the technical jargon went way over my head, it did seem to make sence, at least when compared to bi-wiring. But it would almost double the cost of the wire and it would be quite a bit thicker. Monoprice (and others, I'm sure) sell two sets of wire in a single jacket for bi-wiring/amping which would do the trick. But I don't know....