No, a white female betta will not show any horizontal or vertical stripes when ready to breed. She will, however, still plump up with eggs ... her belly will get big, and she'll do about a 45 degree head down angle with her body when the male approaches her.
When using a darker female, the horizontal fear stripes will change to vertical breeding stripes, in most cases. With all females, no matter what color, the head down angle and the growing tummy are the most significant signs of being ready to breed.