It all depends what you aim to achieve.
Sat very nearly bolt you'll be comfortable but not very areodynamic.
A completely flat back is the most areodynamic but not very comfortable.
If you're on the drops then its probably for areodynamic reasons. In which case your back should be as flat as your flexability will allow. No point riding with a flat back if it hurts / you can't breath!
If you're on the hoods a good comprimise between areodynamics and comfort is in order.
If you're on the flats then you should be sat up opening your chest for climbing / as comfortable as possible.