The USDA recently updated the zone maps due to our warming climate. In my zip code, we moved from zone 6a to 6b. The average low winter temperature in 6b is 5 degrees warmer. A 5 degree shift in temperature does not sound like much, however as plant collector, I may have more confidence buying zone 6 plants than I have had in the past.
You can find the zone for a plant you are purchasing on the plant tag. I find this information helpful as I like to travel long distances for specialty nurseries and buy online - usually outside my zone. Check out the updated USDA Plant Hardiness Zone Maps and be sure to check out the zone for your zip code as the zone can vary from one town to another.