@Mor - It depends on the pendant. If it's particularly large, you'll end up with it being half on the neck and half on the cloth which usually doesn't look all that good. If you've got a dress that sets off the pendant, it can look particularly striking when it is displayed on the neckline.
A jewel neck wedding dress is generally ideal as well, because it's fairly modest and traditional, but you can still make it beautiful and unique to your personality with different skirt types. It doesn't suit every body type though, so I would definitely take that into consideration.