when is it necessary to shim the diffs?

[blueb8llz]

ive been running a 7xl 4s with no probs, but decided to add heavier diff oil for the front and rear on my 3905 emaxx. im going with 85k front and 50k rear. stock is 30k. do you guys generally shim when you raise the wieght in oil, or when you go brushless, or only when you change out your diff case to another? or why? do we have to buy shims or does it come with shims from the factory?