I've always used the maps as guesstimates. They get me in the right box. But the boundries of a zip code are not going to be climate boundries. My zip code has elevation changes of a couple thousand feet - the west side is in the mountains and I am on the edge of the valley. The temps are probably 10 degrees colder there than here.
And then there is your own yard's climate. Buildings, fences, mountains, rivers all affect your climate. Are you on a hill top (warmer), hill bottom (colder), flat spot? Look for microclimates within your yard. My backyard is flat but it sits on top of a 10 foot tall rockery wall. It gets east sun so loses the sun about 1pm. But, its very warm compared to the west facing frontyard where there are no hills, or walls (and is also the direction the storms come from). But my front and back yards are both in Zone 6.
What I am attempting to say is that there are no easy answers. You have to figure out what temperature range you are in. According the the "maps", I am in zone 6 but I have a 4 year old chard plant growing on the south side of my house. The minimum temps for chard is about 20 degrees. Don't tell my chard - our low last winter was 7 degrees and the year before that was 5 degrees. This winter, the chard has been buried in a snow bank for most of the season (It doesn't usually snow here).
Never an easy answer...
