I use this to monitor the situation:
http://www.lowes.com/pd_396632-53921...ductId=3830957
I found this recommendation:
http://www.archives.gov/preservation...ly-papers.html
But Googling turns up several variants - I'd be concerned about a RH below 30 or above 60.
Given I keep the cards in an air conditioned house, I don't find the humidity gets that far over 50 even in the humid months, but I still use this to knock it down some:
http://www.amazon.com/Eva-dry-Renewa.../dp/B000H0XFCS
I'd do some testing and if I found the bank was letting the RH get too high because the AC was turned off at night or whatever reason, I'd think about taking my cards out at least during humid time periods. Lack of humidity in the air can cause the cards to get brittle, but for my climate, I don't think that's as much of a concern.