If you have some golf balls or fake eggs, placing them in the nesting boxes will help with knowing where to lay. I have a fake egg in each box. My chickens who started to lay have only ever used the nesting boxes
You can put them in right now if your birds are roosting without issue, or wait a few more weeks if you wish. I've never had issue with young birds sleeping in the nests, so I don't see a reason to delay putting them in myself.