The instructions in mine say to avoid the early morning and that time doesn't matter as much as doing it at the same time every day but that 2 - 3pm is optimal. It also says to reduce your liquid consumption for 2 hours before testing.
I actually prefer to test twice a day, I don't have much of a surge and I think I'd miss it sometimes if I only test once a day. So I aim for 12-1pm (around lunch), and then again before bed. But I've also heard that around 2pm is the best time. HTH!
I had heard that it should be done between 2pm and 10pm, but I always get my clearly positive opk in the morning (first pee).Later in the day it isn't as positive - I must surge earlier in the day,or perhaps the night before?