Although zone designations have changed it is important to realize that zone designations are based on average temperatures. There can always be a year when we get a polar vortex” like several years ago that kills all the marginally hardy trees and perennials.  Those plunges in temps if they are short don’t change the zone but they do a lot of damage. 
Another consideration is that the Arb has many “favorite trees” home owners have room for one or two and it’s heartbreaking to loose a tree just about the time it is becoming a prized specimen. The impact of loss is so much greater. 
I still consider planting zone 5 material risky and companies that guarantee their plant material do too. Zone “pushing” is fun but it can be expensive if the plant material has to be replaced.