It's that time of the year...everyone is getting new skis and wants to know where they should mount them. To attempt to cut down on all the threads asking about how to ask, I'm going to make this one.
First, you need to understand the difference between core center and true center. Core center varies for every ski, it's the recommended mounting point from the manufacture. It takes into account the balance points, flex, side cut, and height of the sidewall of the ski and that determines where core center is. True center is simply the measured center of the ski, take a measuring tape, measure the full length of the ski from tip to tail, half the measurement and that's true center.
Secondly, what kind of sidecut and dimensions does the ski have? Is it directional or symmetrical? If it's directional, the core center mounting point will most likely not be true center, whereas a symmetrical ski will have the same core center and true center point.
Thirdly, what kind of skiing do you plan on doing? Park, All-Mountain, Powder, Big-Mountain, Trees, Racing...it all makes a difference. Most likely, if its anything other than park, core center is the way to go...however, sometimes people really enjoy true centering certain powder skis.
Now, where to mount? There are several different theories for all different skis. People swear that -2.5 on AR6s is the ONLY way to go...but what you need to realize is that it's 100% down to PERSONAL PREFERENCE. What's right for you, might not be right for someone else, and vice-versa. Now, It's my PERSONAL theory that there only really two places to mount a ski...Core Center or True Center. If a ski is designed to be skied at it's absolute best at Core Center, there's only 1 reason to mount anywhere other than there, and that's that you plan on doing tons or rails in the park and that you are willing to sacrifice some all-mountain performance for the added balace and control on rails. So, I feel that if you're going to mount away from core center, you might as well go all the way to true center. Now, I know that people will disagree with this opinion, and that's fine.
So, the answer to your mounting question is pretty simple. It's really up to you, where do you feel most comfortable riding your skis. If you plan on spending most of, in not all, of your time in the park, and most of, if not all, of that time on rails, you might as well go true center. If you plan on spending most of, if not all of, your time skiing all-mountain, go with core center. Other than that, you need to decide, understanding that if you do decide to mount other than core or true center then the closer you are to center, the more park orientated the ski will be...for example, if recommend is -5 from true center, and you ski 50% park, 50% all mountain and you don't want to go true or core center, maybe -2.5 is the right place for you, lets say you plan on going 75% park, 25% all-mountain, and again, don't want core or true, maybe -1.5 is right. Again, its all personal preference, to be completely honest, NO ONE can tell you where to mount your skis, cause there's a 90% chance they don't ski the same way you do, so they can't tell you what's right for you.
Now that you're decided where to mount your skis, I HIGHLY recommend you do the following, no matter where you plan on mounting them, whether it's true center, core center, or somewhere in between (or maybe even away). First, measure the ski and your preferred mounting spot yourself and take a piece of masking tape and put it around that area, then, measure again and take a marker and draw a line on that piece of tape. Now where you go to the shop you can say "mount them here", no questions, and they're more likely to not mis-mount your skis.
Hope that helps, and if you still have questions on where to mount your skis, post it in THIS thread, that way there aren't a ton of threads asking virtually the same question.