Interesting. With my Nirvana plants I've found topping to actually induce preflower formation. The first ones invariably show up in the last whole node. cn
Crazy... i wonder if we top in a similar way... as far as where and when.
I prefer to wait until after the plant shows its preflowers, but with feminized and schedules to keep up with I sometimes do it before.
When I top for the first time it is to take the first clone that I use as my test for that plant. Usually leave 2-3 sets of branches, not including the lowest branches that I often cut off earlier on.
I feel like I have experienced the opposite. I'm wondering, is your topping a pinching out of the main growth tip? I could see that hurring along preflowers.