The only generic advice that makes sense is to do what it takes to make sure the expansion plug is in the position to support the clamping force of the stem. If that requires the pre-load device (that screws into the expansion plug) to be flush with the stem, do that. If, like my most recent fork, it requires about 5mm spacer above the stem, do whatever makes sense for that situation.

I think the fork I'm talking about came with those sort of instructions.