I second that for another reason. On 9/11 I was in Owensboro, KY and had about 1/4 tank of gas. I didn't feel like gassing up and just wanted to get back to my shop in Evansville. With the mad panic of people trying to buy gas I almost ran dry before I could find a station that didn't have a huge line at each pump. Now I NEVER let it get below 1/2 tank.
I get nervous at half tank in either trucks. I try to keep them full so when I see gas prices fall putting a quarter tank in is only a nip at the bank account.
Also, 2 full tanks in a crisis plus my reserves buys us a lot of time.