Not entirely true what these guys say (sorry guys ) Ive found that on some cars and dependant of the brake discs, the brake disks can actually move them out a couple of mm each tyre very minimal tho..
I know that on my VDF I think the 12mm hex is 4mm deep and my 3 racing discs are definately 5 or 6 I cant be bothered pulling em out.. but it can make a minor difference. ( could be the difference between rubbage an no rubbage )
If you want to increase the tracking width of your car, Tamiya make some 10m deep hexes but you will need to get some longer outdrive cups, I used to run these on my Cyclone for when I was running mega wide bodies.
You could always use a spacer or washer before you put the wheel on to just make sure that you have enuff thread for the nut to hold onto. Or wide offset rims will do the trick, 9mm, 10mm, 12mm's.