Blood pressure does vary from time of day and with activity, so we recommend that when looking for blood pressure as a measure of treatment then review at the same time of day.
Taking diovan is designed to reduce blood pressure but it is not able to give constant control as other factors such as nerve stimulation come into play. Diovan is designed to be taken once daily so taking it in the morning is fine unless you are waking up with an elevated reading, and in that case taking at bedtime may be better for you.
What is your highest daily reading when at rest (that is not after exercise or coffee)?
If at rest your levels are 120/75 then the diovan is doing its job of reducing hypertension. I would stick to checking your levels once a week at the same time of day, and only look to check daily if you were feeling unwell. Unless your blood pressure were to drop to below 110/60 then I would not look to reduce the dose and at the moment as long as your rest BP is below 135/80 then there is no need to increase. And again, there is no need to change the time of day.