I can certainly concur with gingernut's post. He has summarised the current advice very well.
I would definitely say that white coat hypertension is a well documented issue but as GN says, while your BP may be normal, if it goes too high under stressful conditions, this may in itself be cause for concern.
Calibration of these devices is woefully poor in many situations and some of the automatic machines are not very accurate. Doing BPs with an old fashioned mercury sphygnomamometer is the most accurate but most places have banned them due to elfin safety issues!
Getting the cuff size right is also essential. At my last medical the nurse found my BP to be high but when the doc did it with a bigger cuff it was normal. Small cuffs over-read and large ones under-read. The standard adult cuff is probably too small for many larger adults and so they might be inaccurately defined as hypertensive
All in all, quite a crude measure but a useful one taken in combination with other things.
HTH
Bob