People shouldn't worry about accuracy too much. Everybody in healthcare knows about the problems in various measurement methods as well as individual responses to measurement anxiety and the physical and emotional state you are in that particular time. The only accurate measurements are direct measurements through an arterial catheter which is a very invasive procedure.
Routine clinic measurements are used only to have a general idea about trend and secondly to catch severely high BP which is usually due to a secondary disease.
Also, if BP is high above a certain limit it is significant regardless of if you took rest for 5 minutes and other precautions. Because if BP is high the normal BP regulating system should kick in and lower it regardless of the cause. With advancing age and hardened arteries this response becomes less optimal and you need the support of anti hypertensive medicines. You can see this in real time in patients under anesthesia. A young healthy patient would have an initial peak in response to pain or other surgical stimulus but they will be able to lower it either spontaneously or with minimal outside intervention. Whereas in older individuals much more effort is required to control and lower the BP.
The general trend has been to treat both hypertension and diabetes early because the microvascular complications start much earlier before they become apparent clinically.
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