I live in SoCal, so I, like Wayne, don't have to worry about freezing. But I've gotten in the habit of storing my tank empty under the expectation that any residual water would evaporate shortly thereafter, primarily through the vent hole. There's not an easy way to check if that is actually happening, though. We usually go out about once a month, and always leave the house with a full tank of water. We too are in earthquake country, so Wayne's thought about having some water in reserve isn't a bad idea. If an earthquake hits, and you don't have water, you are obviously too late to get any, especially if you need to quickly evacuate.
My immediate thought is that I don't really like the idea of water sitting in the tank for that long, as stuff might grow. But then I just think about all the gallon bottles of distilled and spring water in the grocery store that sit for months at a time. Clearly, that water has not been sterilzed, nor is it bottled in a sterile facility, and you never see stuff growing in the store, or even if you leave a few jugs sitting around the house (which I've done before without giving it a second thought). It's probably filtered, but the stuff coming to your house is purified too.
Dave