How tidy does my room need to be for a furniture preview?

It does not need to be tidy at all. The AI places the new piece into the room as it appears in the photo, clutter and all. General untidiness does not affect the quality of the furniture placement.

The one thing that does matter is visibility of the target area. If objects are blocking the wall or floor where you want to preview the new furniture, move them before taking the photo. A pile of items in front of the wall where you want to see a bookshelf will reduce the quality of that specific placement. Clear the target area, not the whole room.

Everything else in the room can stay exactly as it is. A realistically occupied room actually gives the AI more context about the space: the existing furniture, the floor color, the wall color under normal light. This generally improves the accuracy of the preview compared to an artificially empty or unusually staged room.

Take the photo in whatever state your room is in. Just make sure the target wall or floor area is clear. Try it free on your own photo.