I usually fill up at the Arco close by, which is the cheapest (other than Costco), and has the most convenient location. It's a little annoying because the credit card price is ten cents higher than cash, so I use cash - which means going inside to pay, then going back to get change. But I don't need to fill up very often. If I'm at Costco and the lines aren't ridiculous, I'll fill up there occasionally. Always fill up in Washington rather than Oregon so I can pump the gas myself.
I probably should use an app on longer trips, but I haven't. During our recent drive from Portland to Phoenix through remote SE Oregon, NE Nevada, Utah and Arizona, sometimes you get whatever gas you can regardless of price. I always fill fill up in larger towns, but sometimes I need gas in a place like Fields, Oregon or Baker, Nevada so I have to pay the price - generally when I do a lot of driving in the parks and forests.