if you get your heelpiece mounted correctly, you could get it mounted at center for the new boots you get at first and talk to the shop about putting your heelpiece at the front of the heeltrack so you can fit new boots when you remount. then if you get new boots they can just readjust the heel without having to remount. This won't affect you while skiing. However, if your heelpiece moves back, you won't be directly at center. It shouldn't be a huge issue though, AR7s aren't symmetrical, and you probably won't end up moving back much more than a centimeter, two at the most, and that's if your feet really grow.
I say go for it. Get them mounted center with the heelpiece as forward as possible, and then if your feet grow, get them moved back. if your feet don't end up growing, you have some sweet center mounted skis. if they do grow, you'll have some sweet skis still on their first mount at -1 cm or so.
The only thing that's warm right now is my dong.
Well it's kinda sore, but it was sore before so it's not any worse.
-my friend after breaking his shin in half