Swings are not good, either temperature or humidity. If you can't keep it stable than an average will have to do. Depends on how long it's on the low end of the required %. Most birds in the wild require that the moisture in the ground, if (ground nesters) be around 30% if any lower they will not hatch, and to much moisture is just as bad.
Too little moisture and the air cell will be huge not much room for the chick to develop, and possible won't hatch (DIS) too much moisture and the air cell will be too small the embryo will most likely not develop fully before hatch day, also resulting in a DIS chick or have 'water belly' if it does manage to hatch.
