There is no hard or fast rule about how to wear your hair. Firstly do you personally think you'd look better with your hair up or down?
Then look up magazines of styles that you like, and ask your makeup artist if it suits you.

Also, I think it will also have to match the mood of your gown too.
Anyway, I still have not decided neither for my own wedding! Hehe...I'd probably wear my hair up in the evening and let it loose during the day.