depends on the strain. I wait till they start to turn yellow at the end usually the bottom is ahead of top anyway a selective pruning in last 3 wks 20 mabe30% won't affect outcome negitively Remember shes gotem for a reason.
If your light isn't bright enough to penetrate the lower parts of the plant then I
would suggest clipping off those lower branches during late veg/early flower (unless
you don't mind the popcorn/airy buds.) This will also improve the size of the buds left on the plant.