If you do it the other way around you may have to backtrack if subtracting a less significant digit from another worked out to <0. You will also have to be care

If you do it the other way around you may have to backtrack if subtracting a less significant digit from another worked out to <0. You will also have to be careful about alignment, since in one number you start from the first digit, but in the other you may start from the middle. It's slightly more complicated than just starting from the least significant digit.