I try to always keep the gas tank more full than empty. (Well, except during a road trip - then it gets filled up when rational, and topped off after the trip. I do try to keep at least a small reserve at all times.) It's a preparedness thing. So I fill up at least once every week, more often if I do anything more than the minimum (work & groceries) amount of driving. I don't think it's such a big thing to swing by a gasoline station, and it's going to cost you just as much whether you fill up half a tank once a week or the whole tank twice a month. Might as well keep the tank full. That way you'll always have at least a full tank of gas in case there are problems with gasoline distribution, power, or whatever.