Yuri are correct there is no rule....but...
If some one asked you to place them at the top, after the stack and that leader became known would you do it? Or would you say "there is no rule." What if the player asked this way "as a favor to me, I know you don't have to, but now that your stack and leader is known, would you keep the leader on top?" Would you?
Note I have played over 174 different* OPFOR (not that much by the likes of Steven, Bendis or the Bishop), only 1-2 did this, and I don't play them regularly at all, or even ever again, just by how things pan out at events, so no big deal...but all whom I do play regularly, place the leader on top after the stack is known [there are exception like at set up to avoid letting players see where leaders are before pre game concealment which is super fine], 98% of those at tournaments also place leader on top after "known stack".... {this topic might make an interesting poll...where is Honza? }
I, like Steven, Bendis and Bishop, do think there is a polite feature to doing this, as well as a practical one.
As an aside, I do consider other folks touching / rifling my known stacks as bad etiquette...ask I will tell you what is in it...or I will show you if you don't trust me. I don't like this touching b/c the OPFOR may have forgotten that the last counter of a particular stack late on Turn 5 after 3 hours of play and concentration was concealed and now they rifled through and found the 548. Not cool.