Those machines are AWFUL, particularly in individuals with an appreciable amount of lean mass (i.e. muscle). The best time to check your BP would probably be in the morning, once you've woken up, and before you've eaten anything. BP rises simply with the act of standing up, so taking random readings at random times are likely to be unrealistic.
I have had my BP taken at the doctor's office using one of the machines after I'd just walked up the stairs, and it was hypertensive. Five minutes later, done by hand, it was "like a teenager's."