Yep, my monitor instructions say the same thing, I took my bp at odd times and a lot during a week long experiment to see how much bp fluctuates during a day. The doctor I was seeing at the time (not any more) told me that I had high bp and wanted to put me on captopril to treat it. I thought he had rocks in his head :), so I decided to see if I actually did have high bp or if it was high when he took the readings in his office because he made me so P'd off.
Turns out my bp is perfectly normal, and if I had taken his word for it I would be taking an unnecessary drug for a condition I did not have.