Typically, rear seat, rear facing in the center is safest. However, the owners manual page 1-24, at least on mine says, "Do not secure a child restraint in the center rear seating position using the LATCH anchors. The child restraint will not be secured properly." So, I guess you might be able to use the center seat if you're using the seat belt.
I had my daughter in the rear center of the X for almost two years. When my second daughter was born I was trying to figure out how to keep one of them in the middle and couldn't figure out a good way to do it which is why I looked at the manual. I was surprised to see I had done it wrong for two years. My other vehicles do recommend using the middle but for whatever reason on my generation X it says not to.
Just want everyone to be safe!