D(C)--it's a customizable fit thing, which means it should be customized for you and you alone. There is no universal "best" answer.
But it is a critical adjustment for that stiff boot (I've been in the same boot for a couple seasons). That simple shim affects several things. Number 1 is fore-aft alignment, which is especialy critical with a stiff boot. (This issue was much discussed in the current thread, Best Bump Technique
, which I strongly encourage you to read if you have not already.)
It also affects the angle of your ankle joint. If you lack sufficient range of "dorsiflexion" (bending your ankle forward, toes toward shins), having the shim in place can cause all kinds of problems, including collapsing your arches due to over-pronation, which can cause knee problems and contribute to edging problems.
And, of course, it affects fit.
I'm the opposite end of the spectrum from Steve (SSH) with my skinny calves. I need the spoiler shims in there, and near their maximum (lowest) position. I need them not only to take up space, but also to increase the effective forward cuff angle a little. Without them, the Dobermans put me way too upright, which causes havoc in bumps and any high-performance skiing. For Steve, on the other hand, the shims give him too much
forward lean, which is why they took them out.
Frankly, that boot is one that pretty much requires some professional setup if you really want it to work right for you (not to mention fit!). Good (which is to say, few) bootfitters will be able to answer your question when they see you and work with you in the boots.
Best of luck! It's a great boot--one of the very best once you get it dialed in just right.