rogerd - 3:40 pm on Jan 17, 2008 (gmt 0)

As others have noted, businesses automatically append the +4 for all mailings. Consumers rarely use it. At one point a few years ago, there was a rumor (probably an urban legend) that manually writing the Zip+4 on a piece of mail could actually slow it down (vs. just the 5-digit zip). I forget the reason - perhaps more difficulty in auto-scanning at the time? Seems unlikely.

Then again, how much consumer-to-consumer mail is being carried these days? I haven't written a letter to an individual and sent it by postal mail in ages. I probably mail dozens of items each month, but not one out of a hundred has a hand-written address.

From a business standpoint, oddly, I see the Zip+4 on a relatively small percentage of the business cards I get. Maybe 10% max - the rest all go with the simple 5 digits. You might expect businesses to be more sensitive to the issue, but that apparently isn't the case.