So I finally have one of them fancy dehumidifiers that
has a finely
 adjustable humidi-stat in the basement to keep the collection dry. 
Required listening, the Traveling Wilburys "Cool Dry Place".
  What's the ideal setting? I know too dry isn't
a good thing, for neither
 the collection nor my house. Right now I've got it set to 40% and it's
 running pretty aggressively. My basement hovers between 55 and 60 degrees
 Fahrenheit throughout the year, and the dehumidifier figures out relative
 humidity for you. 
I think 40% is a little too aggressively low and my dehumidifier would run pretty much
continuously May-September if I had it set that low. 60% is a better ballpark and only
during the most humid parts of summer does my basement dehumidifier run more than a few
hours a day.
I will sometimes go as low as 30% setting during the summer but only when I want to
demonstrate my Wimshurst machine :-)
Tim.