Probably a good decision to leave them centered. Since I'm planning on buying some PR's, this thread got me thinking about binding location also. I skied with Peter Keelty a half day in Deer Valley last week, great guy, incredibly knowledgable about skiing. He skis on the 175 PR and loves them, but I forgot to ask him about binding placement, so I emailed him and this is what he had to say about it.
"My PR's (with a 320mm boot sole) are mounted in factory position and seem just fine. Several of our test skiers mount theirs a bit back and claim better deep snow float, but I haven't gotten around to changing mine and so cannot make a direct comparison.
I suspect you've hit upon the best answer; use a binding that can be moved back and forth. Atomic's Variozone models and Tyrolia's Railflex are the moveable bindings of choice at the moment, although we hear Marker will introduce one this spring.
Hope this helps, see you at the Bears' Gathering...
By the way, he has a great web site, www.techsupportforskiers.com
, well worth the small cost to join, plus you can email him questions.