Hmm it depends on what ski you are mounting, i think for now true center mount is the best cause it's just very good for rails, you've got the best stability.
1cm back is not going to make any difference at all, just go center. You could also use the same argument against going center, but why not just go center if you are that close?
core center is the center of the thickest/strongest point of the ski. It doesn't always match the true center, which is the exact middle of the length of your ski. You want to mount true center. Just have them measure the length of the ski and put the binding exactly in the middle of it