TheCrimsonK20 speaks the truth. Humidity really only causes problems on the high end. Besides that it'll only affect the rate of transpiration and, thus, frequency of watering. In order for CO2 to enter through the stomata it has to be first mixed with water at the plant surface and turned to carbonic acid. If the humidity around the plant is too low to complete this process the plant pulls additional water from the root system.
Conclusion: low humidity = water a bit more frequently to make up for increased transpiration.
