Thanks for all your replies! The reason I'm doing it before jaw surgery is because:
1. After jaw surgery, the projection/length of my nose is not going to change from the profile, only the width and the amount that my nose is upturned will change. Thus, if I have rhino considering this fact, it is likely my nose will look ok after the jaw surgery as my nose is not wide at all or too upturned. I will ensure the surgeon doesn't show too much nostril from the front to avoid a pig-like nose after jaw surgery (although I think an upturned nose is cute), and I can even ask him to make it narrower.
2. Furthermore, I've seen quite a few, around 5-6, photos of similar surgeries to mine, or even more impaction, and I wouldn't have changed any of the 'after' noses I've seen so far. I thought they were fine.
eg
http://2.bp.blogspot.com/-2n5yEAiTN04/TXpUS-RmkLI/AAAAAAAAA4E/iK7b1XtcdhY/s1600/before%2Bduring%2Bafter%2Bfront%2Bno%2Bsmile.jpghttp://1.bp.blogspot.com/-Sj5vq4Vi72U/TXpOwsMazGI/AAAAAAAAA3c/JM_wheMcbRM/s1600/Before%2BDuring%2BAfter%2BFront%2BSmile.jpgThis girl already had an upturned nose, and had 5mm impaction and hers turned out fine. As I said I haven't seen one bad case.
3. A further point is that I've heard there is not only an alar stitch to make sure width doesn't increase too much, but also another stitch to make sure the nose doesn't upturn too much.
4. Since I have 5mm of gum show, there will be around 2-3mm maximum impaction, and this most likely won't dramatically change my nose.
5. I want to look nice for the start of university
6. I live in the UK and it takes so long to find a good jaw surgeon, and if I go on the NHS it will take so long. I just don't want to wait another 2-3 years.
So altogether, I will either want minor rhino after jaw surgery, or more likely for me as I'm not looking for a perfect nose, none at all.
Also, even a slightly deviated septum doesn't bother me too much if it comes to that and as I said if I really don't like it I can do a slight revision.
Phew sorry for the long post, I've just thought this through and still want to go through with rhino before jaw surgery.