Costco that's interesting I'm a member but never buy the gas.PaiN is probably right that IF you use Top Tier (http://www.toptiergas.com/retailers.html ), premium fuel you shouldn't have a problem. Even if you use Premium octane, you can have poor grade fuel that can cause unwanted buildup. I know this first-hand in my twin turbo Direct Injection car (learned it after it starting running poorly)
Seafoam has helped clean it out (no carbon/soot out the pipes at full throttle, smoother idle) after one tank.
I've been using StarTron for the last couple of years, every few fillups and over the winter, sitting in the tank (as opposed to StaBil in the olden days, and later a winter or two with SeaFoam). It comes in several concentrations, so pay attention to the label when dumping a bit in the tank.Use StarTron it's formulated to deal with the residues from ethanol most others aren't. You can get it at most autoparts houses and I think Walmart.