I'm on the 'it won't do squat' side of the argument over this.
Considering that people will just fill up with gas on the day before, the day after or something similar to that, it won't do a thing.
If people really want to affect the oil companies and gasoline prices, they need to park the car completely at least one day a month, and more like one day a week. Hmm, more on that in an article I'm brewing up now.