It's almost moot, but to join in I'll go ahead and say this: An idling engine has no load. Load creates higher cylinder temperatures which in turn better utilize fuel, meaning the fuel is more completely combusted. No load results in cooler cylinder temperatures and incomplete burning that could result in unburnt fuel being exhausted. I'm in agreement that the O2 sensor monitors the exhaust quality to adequately protect the cat converter. Idling is something I wouldn't worry too much about.