I don't believe this is a genealogical question, it refers to mailings, so here's what the usps says:
Accuracy of Address Correct address affects efficient delivery of your mail. ZIP + 4 (http://www.usps.com/zip4)code helps direct your mail. Use our Acronymn & Abbreviations (http://www.usps.com/ncsc/lookups/usps_abbreviations.htm)list for a complete address. You can also use our Address Quality products, (http://www.usps.com/ncsc) services and publications to help improve the accuracy of your address list(s) while maintaining your cost to mail.