Hello! Thanks for the question. The time is to allow sensitizer to penetrate the paper slowly, and as homogeneously as possible. The humidity levels of the next step, hydration, have some effect on the importance of this 5 min. "air drying". If, for instance, your are hydrating in an atmosphere of high humidity (anything greater than 50% RH at 20C) the 5-minute air dry becomes quite important.
Now, there is a huge difference between a 5 min. air dry and a 12 hour air dry (coating the night before). The coated paper won't explode, metaphorically speaking, certainly.
But the results will be
different from a 5 min. air dry prior to hydration. Much depends also on the atmospherics during this period. Extensive (anything much longer than 2 hours) storage of coated paper at any stage will alter the results, and ultimately degrade the maximum density, resolution and contrast, as well as lead to unpredictable results.
In sum, this is all about establishing consistent workflows in order to arrive at a fairly predictable result. So, if it is more convenient for you to "coat a sheet of paper, let it air dry overnight, then move on with humidification and the rest of the process" then by all means do that, and do it consistently. Then calibrate your negs for this workflow. You should be fine! I hope this helps! Thanks again,
P