what does your dress look like? your hair should compliment the style of the dress.
a good hairdresser can make a simple half-up/half-down 'do look really really nice...you can go either sleek and straight or have like long waves or curls and pin up the top...i did that, sort of, for my formal. only really loosely pinned.
this sort of style i think compliments floatier dresses, or shorter dresses...with a floor length dress i think it would be better to have your hair up - same goes for a strapless dress. but if you're wearing a knee length dress or a really floaty, girly dress, the halfup-down style can work really well.