In general, humectants work best in middle dew points. If the air is very dry, like in the winter, the humectants will pull moisture out of your hair and release it to the air, drying your hair out. If the dew point is too high, the humectants will pull too much moisture out of the air and into your hair, poofing it up. The middle dew points of spring and summer usually provide the best weather conditions for the most products.

That said, it's different for everyone and the only way to know how your hair will respond is to experiment. Some curlies love humectants in the summer. Some only love certain formulations when the dew points are high (like me - I can use conditioners with humectants in the summer, but only if they're toward the middle or bottom of the ingredient list, so it depends on the product).
Sulfate/non-water-soluble silicone free since 8/16/10
Northern VA
Med-coarse, normal porosity, low elasticity
Dye-free since 11/2010