Well, Costco is a reasonably big operator. The nature of things pretty much requires that they at least be the equivalent of the worst high volume brand name gas out there, which is probably not too bad. Since they kind of position themselves as offering luxury on the cheap for their other products, I wouldn't be surprised if they go beyond that, if the market demonstrates it values extra high detergent gas.
I've actually toured a refinery that supplied Chevron and a large regional gas station chain. The only difference was the tank that dumped the detergents into the Chevron as the trucks left the refinery. There is no difference in price between the regional and Chevron--the customer who thinks its all the same just gets short-changed. I haven't stopped at the regional since.
The absolute worst are the diesel stations along the interstate. In my area, its common for transmix--the slug of liquids between an order of gas and an order of diesel along the interstate pipeline that is both--to be captured naptha added and then sold as diesel instead of being rerefined. If I owned a diesel I would be highly paranoid about fuel quality.
On the gas side, the ones I worry about are the complete no-name low buck guys with one or two outlets. There was a time when I would automatically go to whoever had the cheapest gas by two cents and not think about it. No more.
