Unless it's really bad, you shouldn't have to worry too much about the orings on your float bowls. The seals are not under any real pressure, and shouldn't have excessive amounts of fuel against them in any case. You've probably got it clean enough. That being said, nothing wrong with being diligent in your cleaning. Don't scrape the areas though, as a gouge in the soft metal will be every bit as much a leak opportunity than a little left over rubber is.