Not knowing what type of hem it is, or style and fabric, it's difficult to answer. But as a general guide, I'd level the hem while the bride is wearing it (in her bridal shoes). I usually use a small cotton bobbin to mark up from the ground, by putting a pin in the dress every few inches, at the top of the reel. This ensures it is the same distance from the ground all the way round. Then, determine where she wants the finished length and measure down from the pins for the hem edge.
How you finish the hem depends on what it's like originally. I usually copy whatever it was originally, unless it looked truly awful and I could do something better. I use this method of levelling on bridesmaid and prom dresses too, as they can be so uneven and there's no way of telling that when it's trailing all over the floor. Much safer than taking the same off all the way round.
Suzie