I recently bought a Blichmann Therminator counterflow chiller (I know, extravagant, but it is the awesome).
The instructions say to operate it with the hot wort input facing down, so that gravity causes the wort chamber to fill correctly. They say that if you get a bubble in the wort, it will oxidize your beer.
Really? One bubble? I don't worry much at all about hot-side aeration when moving hot wort around. Why should I worry about a bubble?