Big +1 from me here! The number of times I've accidentally sent some message to the wrong guy, just because my chat boxes were suddenly in a different order than I was used to...
Right now, the only way to affect the order is by quickly opening all chats in the desired order, right after you join. Then they'll stay in that order, even after closing and re-opening. Which, sure, is one way to do it, but really rather cumbersome 😄