Dorian popularized doin em standing a little more up right than parallel....almost 45 degrees, with a shoulder width palm up grip. You gonna argue with his back development? I love this method. Try it for variety.
I do them about parallel to the floor with a arch up to my head. The only thing I noticed is people who use less of an angle is they tend to end up doing basicly shoulder shrugs and aren't working their backs. If you do it with the right form 45 degrees will work.