I like my smiles to be elegant and I voted just curved, although I may go a wee bit deeper than that. I try to mirror the shape of the nail at the cuticle.
As for deep curves/smiles, I think they look rediculous and very unnatural and not at all flattering on any nailbed. It is fine for competition (I'll need to practice going deeper LOL) to show someones skill but for day to day clients, definitely not.