I always lollipop, no matter how short or tall the plant is.
Defoliation is good to do as soon as you notice that it's done stretching after you switched it to flower. this usually happens in week three, so i like to defoliate then. I remove all the leaves that cover any bud sites, which is easy to tell at this point, due to pre-flowers being present. If the leaf isn't covering a bud site, leave it alone.
You also want to keep feeding it nitrogen through it's stretch phase. So, make sure you're still giving it veg nutrients for the first week or two, then switch to your flowering food regimen.
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I hope this helps!