Actually, I wait to about week 3, showing flowers before I do that. It really makes no difference in production, slows down growth by 1-2 weeks (wait, that can be good), and only slightly lowers the height on an indica.

There are times that I want to make a plant take an extra week or so to finish. I will do a flower room top on those select few. On the ones I have done this on, they were already very stable and it did not ellicit any hermaphroditic response from the plant. Makes no real change in amount of/quantity of product. You don't get the really nice arm sized top main cola if you do this, it all smokes the same.

For my Sativas, they start getting the trimming as soon as they are stable in a 4" pot. I do this to keep down on height. I get oodles of thumb sized nuggets from them, but no main cola.

There are times I wish to delay the finishing of a plant, lets say I want it to go another week to keep my rotation in the proper motion........then I do this to stretch out things a little and stay consistent on the amount of medicine my patient and I harvest per week, which, one plant seems to do us perfectly.