I think OA flux left on the pot would quickly looses its activity and become part of the dross waste stream, and smoke stream for that matter.
Also, if your wave is setup properly your selective pallets will scour (push) the top of the lambda wave prior to soldering. Any contamination then is suspended in the liquid solder bath / fountain (but not for long with the density differences). If it is in fact always there why would it be ok for OA processes and not NC?
I would be more concerned with what is in your bath like copper, aluminum, iron, cadmium, nickel etc. (lead if lead-free).
Also, I have a copper FR-4 board on my desk that was OA fluxed with a very aggressive OA and then wave soldered back in April 2004. It was not washed. Ther is no evidence of dendrites, missing copper, tarnish, patina, buggers, nothing. Is this imperical? NO. I just wanted to emphasize that propper thermal profiles coupled with a reasonable flux process will consume (virtually) all activity in the flux and the wave foountain will scour the remaining residues. However, If I drip a couple of drops of this same liquid flux on the copper plate, there will be missing copper within 24 hours (at room temperature).
I have a dual flux wave. The biggest problem is keeping the fingers clean and the grease in good shape.
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