My leucs sleep up to the very top corners of the tank (they are in a 35 hex). They are on the floor in the morning, but I think that is because they are getting ready for food to be dropped in. Mine like to hide, so as long as you had things they could hide in up top, they will use it.
and make sure they have an easy way to get to the top (ie not just flat glass). I have a pair of tincs that most people said would stay more on the floor rather than climb so i aranged the tank in a way that they had access to all levels without having to make too many cliffhangers.