Firstly 2-3 weeks of using plain water is gonna stress the hell out of your plant. If your growing in soil just reduce your feeding strength gradually over the last 3 weeks so you start the last week on just water if you plan on flushing.
I personally do NOT flush my plants.
The reason for chopping plants after a period of darkness is that THC breaks down in daylight and repairs during darkness. So you will have a SLIGHT increase in THC production after the prolonged darkness.
If you allow your plants to dry out before chopping it could speed up the drying time and cause a harsher smoke in the final product.
Alot of people are now started to try flooding the roots during the prolonged darkness. You literally drown the plants roots in water without the use of an airstone.
Do some more reading.