When I see some amber trichomes, I flush for 1-2 weeks then harvest. I do see a noticeable difference in my smoke (less harsh) by flushing,
I know some think flushing is a myth. I once hit a plant with good dose of Koolbloom dry, then decided to harvest the very next day. The smoke was not only very harsh, but actually made popping sounds while burning from the high level of nutrients. I will always flush.