It depends on hardener, emulsion and hold time. I have seen hardeners such as formalin cause an increase in fog if the emulsion is held for hours prior to coating. This is usually only a problem in production if something backs up the schedule or whatever... With hand coatings, over say 4 hours, I see little or no problem with severe fog but do see some changes including a slight increase in viscosity.

So, it depends. At Kodak we often added hardener in a separate layer just as a precautionary measure.