In southern Wisconsin, humidity can be a problem. I run a dehumidifier in the basement shop from June into November. With the central air, the relative humidity stays around 50% and I do not have a rusting problem or mold/mildew problem. We do not humidify the air in the winter and the humidity van drop considerably lower then. If we get a lasting cold snap, the r.h. can drop into the teens.
Before running the dehumidifier, rusting was a constant problem. During the summer, I will pull about 2 gallons of water out of the air each day plus whatever the central air takes out. I should point out that the house is over a hundred years old and the is no vapor barrier in the walls and the foundation is sandstone/limestone with lime mortar so in in the past the walls would wick moisture in from the surrounding soil. The shop is maintained at a fairly constant temperature year around; aqout 65 - 70 degrees F.
IMO, as far as metalworking machinery is concerned, the less humidity the better.
Bbo