I don't normally humidify my paper, I have for some alt processes, but if you have a piece of paper with a crease or buckle I think humidifying before coating might save the day. Also, coating one side of paper causes it to expand, and then contract on that side more than the uncoated side. Humidifying pretty much solves that problem.
As I said, I don't as a normal routine, but the paper stayed flatter as it dried, and was flatter after fully dried.
I hope Denise Ross sees this; she humidifies her paper sometimes. In fact I was thinking her when I wrote it.