Athosanswered grow question 7 years ago There's no final answer for this, some do it, some don't. I tried it this time around, you are welcome to check my diary.
The idea is to allow light to reach lower leaves and thus promote bud development in the mid-lower region. Light to be useful must reach leaves, not stem, soil or pistils. It also helps to keep humidity down during flower, which can be problematic.
After 21 days of flower (the end of stretch) is a perfect time to do it, you can start light if you are unsure, I did it and didn't notice any ill effect. But careful, you don't want a bald plant with only stems