I use a Philips 66 card with their smartphone app and get .25 a gallon off at their stations. 10 cents for using the app and 15 cents extra for paying with their card. I guess now 5% cash back would be the same.
You will likely get your best "discount" by using Gas Buddy to find the cheapest fuel in any given area. At least that is I what I have found.
LOCALLY, from highest to lowest: Diesel is working out to 7.5% "off" per gallon TODAY. Regular gasoline is working out to 9.5% "off" per gallon TODAY. I have found that, locally, Phillips 66 is consistently has the highest prices on fuel. As of today, there is a 44¢ price spread between the highest in both diesel and regular gasoline. Sam's Club and Murphy Express are consistenty the two lowest priced stations with a price spread today of 2¢ on diesel and 4¢ on regular gasoline between the two.
I also have no problem unhooking to get into a fuel station, particularly if I spend the night in the parking lot. I refuel either at the end of my travel day (I don't drive at night) or in the morning before I head out again. Sam's Club doesn't always have diesel whereas Murphy does. I like Sam's Club and I can usually hit one every night while traveling (I top off my fuel daily which allows me to choose when I get fuel rather than the fuel tank dictating where I get fuel). I get enough "cash rewards" just off food shopping to make my membership free, plus a bit extra. But it's not for everyone.
The only warnings I would give you on Gas Buddy (other than their "offers") is to watch the dates on the reported prices. Some places will have very old dates on them. You shouldn't pay any attention to any price older than 48 hours.
Also you can check the price of fuel in any town, in any state, simply by doing a search (I use the website for that kind of research, not the app). Before you get all excited about getting a fuel chain's loyalty discount card, you can use Gas Buddy to check around and see if the discount is really worth it. So far, my fuel chain loyalty cards just take up space in my wallet. Although they do come in handy for scraping a light frost off the side windows.