I have done it in the past but haven't been doing it recently. This thread spurred my curiosity about how much of a difference it makes so I just broke out the spreasheet and did a quick calculation to see. Here's what I did:

I assumed an initial price of $2.25/gal and a rate of increase of $0.05/week over 27 weeks. I also assumed 200 miles/week on a car that gets 40mpg for a total fuel need of 5gal/week. There are two cars: Car A is fueled weekly with 5 gallons of gas. Car B is fueled every third week with 15 gallons. Here's how it worked out:

Car A: 135 gallons at a total cost of $391.50
Car B: 135 gallons at a total cost of $398.25
Money saved by fueling weekly rather than every third week: $6.75 or 1.7%

**EDIT**
I realized that 200mi/week is a little low so I re-did the calculation for 400mi/week. Car A fuels with 5 gallons twice a week and Car B fuels once every week and a half with 15 gallons. The same rate of increase is assumed, and assumed to be continuous (2.5 cents every half week). Here's how it worked out:

Car A: 270 gallons at a total cost of $779.63
Car B: 270 gallons at a total cost of $786.38
Money saved by fueling twice a week rather than every week and a half: $6.75 or 0.9%

Interesting that the more you drive the less it matters.