You need to place 9 to 14. Most of the times, when you are quite near top server and top server also promotes, finishing 9th or 10th will not be enough to tank. So to be sure you should know when your top server does not promote and tank this season, or else finish 13th-14th to be quite sure.
However, except for keeping your well trained players for a longer time for associations and saving packs/tokens I don't see much sense and fun in tanking.