W.A. is a bit different. Fuel prices are regulated and have to be set by 1400 the day before, submitted and recorded by (whoever is responsible) and then advertised, prices can't be changed after being submitted. A website called Fuelwatch then compares the best prices as per your search criteria - yes, of every store that sells fuel to the consumer in W.A., it's great.
My closest 4 are BP, Woolworths, United then Caltex. (I ignore Gull due to the Bio-diesel and unmanned store - with no price saving despite the decreased staff costs - angers me to tears).
Usually the cheapest are WW and United running pretty close, then Caltex, then BP (often 6 - 10 cents more than WW, although I just checked and today it's only 4c more for diesel and WW is equal to BP for unleaded, but I think the unleaded prices are still in fluctuation from the big drop in prices recently).
I fuel up my landcruiser with 150L of fuel at a time, usually once a month. My wife once a week for 60L. We buy something that we need anyway with no markup - for us it's prepaid phone vouchers, doesn't matter which shop you get them from, they are always the same price. It would work for scratchies too if you a someone who must have scratchies, but probably not much else unless you use your imagination.
I save $12 flat at every top-up plus the reduced price of going to WW instead of the more expensive, but much closer BP. My wife approx. $4 - 8 (x 4 = av. $18/month.) Multiply $30 x 12 = $360/year. Nothing to sneeze at for the extra 120 secs to get from BP to WW.
Sure this only works cause I'm on a prepaid, and our WW is ALWAYS cheaper than our BP and 95% of time the Caltex as well, and usually by a big margin (and you can probably tell from the above that I have the type of personality that checks, not every fuel up, but average of every second one - it used to be every one, but now there really is no point as BP has never been cheaper on any of our checks).
Now this doesn't work for everyone, but my wife shops at WW, Coles and IGA (all in the same week) following the specials - not my cup of tea (but then I check Fuelwatch prices - which I'm sure makes many of you guys groan), and our vegetables come from a local farmer direct supermarket so I guarantee you we're not being ripped off by being a loyal customer to either of the big chains. Works for us and shows that the dockets can work, but only in some pretty extreme circumstances. :-D
All our Coles vouchers go in the bin as the Shell is the next furthest away station and they are never cheap enough to compete, especially against the 8c WW (yes I keep the Coles dockets until expiry, sometimes checking to see if they're worth using, but then we ALWAYS throw them out).
God, I need to get out more!