It has been my experience that women naturally have to wear their hakama higher than men, due to the greater flare of their hips. However, this does not cause the sword to sit more vertical. That is generally caused by having the obi too loose to hold the sword properly. The looser the obi is, the more vertical the sword will sit.
It is more difficult for women to tie their obi as tightly because it sits higher. I have found that the easiest way to get around this, and thus cause the sword to sit correctly, is to ensure that the saya passes over the lower hakama himo after going through the obi. This wedges the saya between the obi and the hakama himo, and helps to keep the sword from falling into a more vertical position.
Paul Smith
"Always keep the sharp side and the pointy end between you and your opponent"