There are many reasons to be aggressive with management of hypertension. Indeed it is a silent killer. Given that setup, it is equally true that it is important to accurately diagnose and there are guidelines for doing so.
A nice review of proper technique including ruling out "white coat" hypertension is available at
http://www.heartfoundation.org.au/Si...2010Update.pdf
A single elevated reading is not usually "enuf" to justify going directly to medication unless it is extremely, pre-stroke, levels or there are other signs of significant cardiac compromise.