I realize this is kind of an old thread, but thought I'd contribute...
While I don't know for sure, I'm gonna place my bet on her ascendant as:
First, my ex wife was a Sag sun, Cancer asc. Personally, I can see a LOT of physical similarities in the photos here and my ex. Incredible resemblance, IMHO. (No, I don't have any pics. Sorry.
I agree with Kannon about the Cardinal asc, which Cancer is.
As an aside, having dated a few Libra rising women, I see zero Libra aspects in her. None whatsoever. Again, just my opinion.
She's got quite a moon-like face. Granted, at 5'-9" she's too tall for the average water asc (my ex was 5'-0"), but genetics do occasionally override. I knew an Aquarius sun, Taurus asc once who was 6'-8". It happens.
Also, at the risk of sounding sexist and perverted, as a male, I have to confess that I've noticed something distinct about the breasts of women with Cancer asc and once came across an article that mentioned it, too, and it has nothing to do with the bra size. With my moon in cancer, I happen to notice these things ...and it's no surprise that I married a Cancer asc, and a Sag sun married this Sag asc. My moon felt very comfortable, emotionally, with the Cancer asc. As well, the girl I'm dating now has her moon in Sag ...she's emotionally comfortable with my Sag asc. Funny how that all works out, eh?
Remember, Cancer is the mother of the zodiac and represents all things feminine and mothering.
In addition, looking at her natal chart we can determine several things about her houses, which we can draw from her life as a public figure...
Sun falls into the 5th house. Definitely the 'showman' (or, show-woman). Loves the stage, obviously. As well, no surprise she married an Aquarius sun (I've seen many 'opposite' energies in marriages).
Probably early Cancer asc, so Moon probably falls into the 3rd house.
3rd house: Communication & siblings.
She's got the acting thing (communication) and has her foundation about her brother (siblings). These are emotional needs for her. No brainer, IMHO.
Also, the moon in that 'Gemini' 3rd house energy is going to feel very emotionally comfortable and at peace with that trine Aquarian sun. (don't know anything else about him).
All her planets would fall in the Northern Hemisphere (at the bottom). According to IMDb, she became a child model at 9 years old. I'm gonna bet that deep inside she feels like she never had much of a childhood ...indicative of people with these planetary layouts. Michael Jackson was this way.
Venus in Scorpio falls to the 4th house. Venus is part of how we project ourselves to the world. More classic Cancer energy here: Home, mother, family. People who know her probably say she's a very nurturing person. Probably has a great relationship with her mother. IMDb quotes her in 2009 as saying: "I'm excited about having a big family. I'm talking five or six." Need I say more.
Jupiter in Leo falls to the 1st house. Jupiter shows us how we grow and 'expand' in our lives. There's definitely some energy that indicates that her growth took place through projecting a self identity, self expression and appearance (all 1st house stuff) in being a model at such a young age. As well, Leo likes to perform...personal growth through performance.
Naturally, a careful analysis of specific, major event in her life could be used to surmise her birth time.
Hope this sheds some light on Katherine Heigl's astrology.
I'm no expert and always trying to learn more. If anyone has any better insight, I'm always open to hearing another perspective.
Hope I have the chance to meet her some day and I genuinely wish her the best.