The beetles are warm weather critters and need higher temperatures to hatch. With biological things there is no absolute ... but temperatures higher than 70F are associated with increased risk of any beetle eggs hatching. That doesn't mean you are perfectly safe at 65F and that doesn't mean you are definately going to get beetles at 75F. I'd be comfortable with 72F as long as I didn't have a particularly valuable stash or rarely opened humidor.
Lowering the temperature in a closed system will increase the relative humidity,
http://www.sensirion.com/images/getFile?id=115, but your humidor is not a closed system, you have a humidification device that will modulate the humidity change. So lower temperature by itself is not a problem as long as you have a good humidification setup. If I had valuable cigars exposed to temperatures much above 70F and I was ageing them (i.e., not looking at them too often) I'd consider freezing to decrease the risk of beetle infestation. OTOH I'd not worry too much with 72F with cigars I'm planning to smoke within a few months.