Winter watering is mandatory for sustaining plant vigor.  Insufficiently watered plants often suffer root damage during winter, and emerge looking healthy, but succumb to disease or insects.  Plants and turf with southern or western exposures are particularly vulnerable to damage.  Water when air and soil temperatures are above 40 degrees with no snow cover.  For additional information check out http://www.ext.colostate.edu/pubs/garden/07211.html