Leave them alone and let then work it out. There are always exceptions, but their instinct is usually to sleep on the highest point possible and the ones highest in the pecking order get to sleep where they want to sleep. As they grow the ones at the bottom of the pecking order may get forced to the lower bar, depending on how crowded they get.
Lots of people use the ladder type roosts where they are aft different levels. The chickens work that out. If you wish you can put in more roost space at the same height. Height is not the only thing that makes a certain roost preferable, location in the coop can affect that too. Mine like to be near a window. The dominant ones will still sleep where they want to and the ones lower down will make do with what is left. They will work that out too. This is not something you need to micromanage. Have enough roost space available higher than the nests and they will work it out.