I know a lot of people that shop at certain stores so that they can get a discount (10 cents or so) on their gas purchases at that same store. It sounds like a great deal, but not always.
If you are shopping at this store already because they are saving you the most money and have a great coupon program, then it's great to get the gas discount as an added bonus. If you are only shopping there for the gas discount you're probably spending a lot more than you are saving.
I mean think about it, if you have a 16 gallon gas tank on your vehicle and you have earned a 10 cents discount per gallon, that's only a savings of $1.60! If you can shop at another store and save more than basically $2.00 then it's smarter to shop at the money saving store.
There are some stores however that will give you a smaller discount on the gas (say 3 cents) just for scanning your store loyalty card, no purchase necessary. I normally do this and then get my groceries at the store where I can get the most savings and stretch my money as far as it will stretch.
It's funny how some "great savings" plans aren't really that great when you look at the numbers. Don't just assume you're saving the most, check your numbers and adjust if you need to.