It's a good question, but it has been the subject of lots of threads here and videos on youtube. What you are talking about is rotating your pelvis and getting a flat back or riding fairly upright. Visually, somone riding a townie with a basket on the front is riding upright, a competitor hammering in the Tour de France or sprinting on the track is riding fully rotated. As the previous poster said, it's up to you. Power, endurance, speed, saddle type, bike fit and alot of other variable are interdependent with how you position your body on the bike. Your sitz bones are triangular, so as you rotate forward your contact area become more narrow. My bias, having your pelvis on a different angular plane than your back is a bad idea for any significant distance. The arched bridged whale back effect is a natural thing for experienced and supple riders, as a noobie don't try to mimick that look.

I have one MTB that I use as a commuter, I ride it more upright with a riser so I can have a better view and faster responses, hence I use a b17 saddle. However, I am still rotated ...just not as severe as a road bike. I think you'll find that your saddle position front and rear should match your rotated position, or your legs will be experiencing too much or not enough reach, which will affect power and endurance. This is a good argument for a bike fit, assuming you can communicate to the fitter the position you want.

You can figure it out for yourself by doing long rides. If you are comfortable and efficient at 60 miles, you've got a workable position and fit, if you have problems they'll certainly show up.