I don't really see the point. The only time anyone would ever be able to even see the piercing is if you had on a bikini or something. And back dimples are noticeable/visible enough, not there's really no point in putting a piercing in them.
I think those type of piercings are called Surface Piercings. They grow out and leave scars like eyebrows, back of the neck, basically anywhere you are in the surface flesh. They are quite notoriously tricky to heal so I've heard as the body's natural reaction is to try and expell the foreign body. Read up on it a lot before deciding.