To add to Brian the Brave's post... Each manufacturer can be different. I DID follow the instructions from DiscItalia. I would have to pull the wheels to look at the vanes in the rotors, but the instruction sheet stated for the rotation to be based on the slot direction. I'm sure the reason they did this is that it's the easier to see a visual like the slots than to remove the wheel and check the vane direction.

Cheers!