While not nearly as much as the other atmospheric conditions, relative humidity does affect firearm ballistics. Humidity affects the air density, tending to decrease the air density a small amount, depending on the relative humidity in the atmosphere and the vapor pressure of water at the temperature of the atmosphere. The effect of humidity is generally greater at locations near sea level on very hot days, but even under these conditions, the effect is small. With air that is less dense the bullet as it travels encounters less resistance and is able to conserve its velocity longer.
If you do not check the 'Correct for Atmosphere' checkbox then the International Standard Atmosphere humidity of 50% will be used.
The Weather Underground is a good website to use to find out the altitude, pressure, temperature, and humidity for where you live. Plug those numbers into the ballistic trajectory calculator in order to have more accurate results.