Flushing is a very controversial topic... After lots of research I came to the conclusion that the common belief "flushing in the last week is essential to remove the excess of nutrients built in the plant, otherwise the weed will smoke badly and harsh etc" is not actually true. Basically because :
1) Excess of nutrients is not stored in the flowers
2) The huge companies growing weed do not flush
3) Nobody growing other cultivars flush their stuff, including the multimillionaire and long established tobacco industry
4) Why would you make your plant starve to death and be obligated to consume itself in the moment she needs nutrients the most (to gain wait and create oils)?
5) What makes weed smoke harsh and burn black ash is improper drying and curing, only that.