Binding position!

Hi, I am in the process of picking my first set of twin tips, however, the binding position confuses me.

If I put the bindings in the middle, I can ski backwards with good control, however, I hear that I would lose control in deep powder (being from Colorado, I do quite a bit of powder). I was also wondering, since the pros mount their bindings in the middle, if that is why they have skis that look far to big for them?

Also, I have heard the term directional when referring to twin tips, how does that work?

Thanks for the help!