I believe it depends a bit. If you're sitting at home and not exerting yourself you likely don't need a gallon of water per day for drinking. Probably more like two quarts. Depending on what you eat a lot of the water can come from food. Of course, if it's very hot or very cold, or if you're working hard...you'll need more water.

It seems prudent to have a mix of stored water and water treatment options. Larger containers are much more economical than bottled water but for convenience a mix of both is probably good. In my current home I have a mix of larger Aquatainers and gallon jugs of water purchased at Wal-Mart. The latter is about $.88/gallon and has a shelf life of a year (and of course, after that it's still water- I can boil it, use chlorine/chlorine dioxide, etc). I don't store as much water right now as I did at my last place but I live very close to the largest reservoir in the state of SD.