I literally just googled "Legging Homebrew" to figure out what you were talking about (what is this new technique I've never heard of?!)... but then I realized you meant kegging (at least I hope so). I would say don't pour it in the fermenter and stir, because you will then kick up all of the settled yeast and what not. When you are ready to keg the beer, you can add the lactose to a pitcher of the beer on the side, mix it up, then pour the beer into the keg
If I were going to add lactose at kegging/bottling, I would treat it pretty much like priming sugar, and keep stirring (if any) to a minimum.
