We are older folks with lots of time and on a very fixed income. We keep track of what cards do us best at any time. While Kroger fuel points are a pretty consistent winner, the 35 gallon limit is a serious factor. We do have POIs set for Sam's Club and now BJ's (not much coverage west) and both of those are consistent winners but not always convenient to the route. We no longer have Costco's because the local stores are too distant now with the above pair local.
Because we co-drive and one of us gets to be a full time navigator with wire usually available through a phone tether, We can search for fuel prices and the go back to out old Street Atlas and figure out if the divergence is cost effective. I have a spreadsheet set for that. (Aren't new laptops amazing?) What I discovered early on was that if you diverge from your planned route just to buy fuel, you have to replace both the fuel to get to that stop at next stop may now have to buy fore fuel. The computer gives us the distances very easily.
And I used to do this all with paper maps and a calculator. Gasbuddy, Waz and out of date Street Atlas really is much more simple.
Matt