Although I usually choose plants for my zone (4) or lower, I successfully grow things for zones 5 and sometimes 6. There's so many micro-climates around a house, that I don't feel I need to know my exact zone.

Besides, most gardening books rarely agree on a zone for a specific plant.

Here are a couple of links that give a more detailed look at Delaware's zones.

http://uncommonrose.biz/r/delaware.html
http://mgonline.com/zm_mdde.jpg

Arbor Day website has a great zone map tool that works by zip code so it can tell you if you are in a microclimate.

Here is a link that might be useful: Arbor Day Org Zone Map Tool