No fixed rule, just depends on the situation and each one seems to be different.
If I know the sites are large enough and the roads are wide enough at the turns, I will wait until I pull into the site to disconnect, that is rare however. I generally will gauge the check in when we pull in and if there are already people inside checking in, I will hurry and disconnect the Jeep then head in to register. The timing usually works out well as my wife and I are walking in, the people ahead of us are walking out. If we are the only ones there sometimes my wife will head in and I will start to disconnect or if the area is large enough, we will just check in then come out and disconnect. To be honest, I can be unhooked in less than 2-minutes and if someone can't wait that long they are participating in the wrong hobby.
Mike