In NZ we have school houses. Remember ... Hogwarts is a boarding school and the role of your house is also your family ... for your entire school life. In that respect I think you have to sort on day 1 . Unlike real families ... you get placed into an environment where your natural inner qualities match other people similar to you. It might be odd at first ... look at Neville from book 1 to book 7, but it's magic and seems to work.

Everyone, I think has shades of light and dark ... so, most people could go to any house. But in the end, I think it's your strongest tendencies that are revealed on the first day before any influence has changed the real you.

But, that's my opinion.