I use the Omron BP786N. Upon the recommendation of my Dr. I take a measurement before I even sit up. The reason why is that the level of my blood pressure medication will be the lowest of the day. If it is consistently high at that reading then my dosage may need adjustment. During the rest of the day your blood pressure will fluctuate between highs and lows, either of those peaks and valleys aren't representative of your actual mean level blood pressure which is what you're treating.
For example getting a reading at your Dr's office isn't always a good representation as you may have stress from just getting to the Dr's office.