Topping seedlings before they have shown sex is a habit worth breaking if space is tight. The stress of a cut around week three tends to push preflowers back a few days, and you end up giving shape to males you will throw away, so I wait until every plant is sexed and top only the females. Data from growers who compared topped and untopped seedlings for time to preflower is brought together on https://breederregularseeds.com, and the delay was consistent. The females lose nothing by waiting a week.