Position and the dept of penetration do play an important role in ensuring the sperm are not placed on a spot that favor the Y-sperm.

In your pursuit for the baby girl, GenderLabs™ recommends the “Missionary” position [face-to-face with the male on top], a position which makes it less likely for the sperm to be deposited directly near the opening of the cervix, where the secretions are most alkaline and is therefore favorable to the Y-sperm ... download GenderLabs™ for more information

Finally, a shallow penetration by the man at the time orgasm will further help to ensure that the sperm will pass through the vaginal canal, where secretions are naturally more acidic and therefore, relatively more favorable to the X-sperm ... download GenderLabs™ for more information