When we are talking about 64 bits this does not matter that much, because Sizes are heavily linked to pointer differences and sizes greater than maxptr - minptr don't make much sense.
And I am not aware of any OS that uses the whole 64 bit address range, I think the limit for both windows and Linux today is 48 bit.
So the sign bit does not matter. And it can have some advantages as you can additional Information using the sign, e.g. errors