I'll respectfully disagree that cleaning is not needed. Good flux can dissolve oxidation that's there, but there's no reason to depend on it to do that. What it can't do is dissolve any actual dirt embedded in the metal. I've sweated pipes before without cleaning them and watched the dirt float up out of the joint as the solder displaced it. The joints held fine, but it took more time to get a good filled in joint because I had to let the dirt flow out and then wipe it off before putting the final crown on the joint. On top of that, a significant part of a soldered joint's strength comes from the roughness of the part being soldered.