I completely agree with Tosh. Stop that! It may use more gas, but the annoyance factor for passengers and other drivers is much more important IMHO. I actually push my heel as far forward as possible, allowing me to have my foot at a 90 degree angle to my shin. That greatly reduces the effort involved to keep my foot steady. On some cars, though, I will rest one side of my foot along the tunnel in the center of the car, as some above have stated.
About the annoyance thing, I tend to let myself get really close to people like that as they slow down, then back off as they accelerate. It's not that I want to tailgate, it's my way of sending a message to quit slowing down. If you do, I'll back off. As a passenger, it makes me nervous. The only thing worse was the one time I drove with this guy who kept letting his car drift right until he was almost in the next lane, then correct it. Sort of the steering wheel version of what we're talking about here. My buddy and I in the back turned 6 shades of pale. Scary.