I suppose it depends on the climatic conditions around your cellar. Ideally the humility in a wine cellar should be around 60 to 70 percent.
If your cellar is consistently drier than that, in theory the corks will dry out, shrink, and allow wine to seep out and, worse, oxygen to get into the bottle, where it will adversely affect the wine (that's the theory -- I've never actually seen it happen, even after living for 29 years in the desert climate of the San Fernando Valley).
OTOH, if the cellar is too damp, mold might grow on the labels, eventually ruining them, and they might fall off the bottles if the glue gets damp. I've seen some of this on older bottles, but it's never been a problem for me, and I've never heard of the mold actually affecting the wine itself.
I live about 500 yards from the ocean, and my air-conditioned cellar usually stays around 60 percent humidity. However, the house is relatively new and thus well insulated and swathed in Tyvek housewrap. Nonetheless, my guess is that your cellar has to be in a pretty damp area before excessive moisture is much of a problem. Still, if I were building a new cellar, I'd include the moisture barrier on the theory that it would help maintain constant humidity.