Well all I can say is, each girl to their own thing. I really do NOT think having a passing female voice is going to help you, if you DON'T look like a natal woman anyway. Both work in tandem, both must be the same. If anything if you DO have a female voice but look a little off, then - if anything, you will still get an awkward look (they might at this point be a little confused) as gender recognition happens within seconds of looking at a face, its naturally ingrained into us subconsciously.
Moving away from that, voices are different ranges from male to female, women can have lower toned voices too, not every girl sounds high squeeky, barbie doll, what people need to do is basically find a voice that they feel comfortable, and is within the female range.
Catherine.