As mentioned previously! if you run your tyres until you've got around 2 - 3mm on the front and then 5mm on the rear and then swap them over front to back, this will be a bad idea as there will be still plenty of grip on the front and not much on the rear resulting the car tending to oversteer, which is harder to control for someone unexpecting it!
if you want to change wheels over every few thousand miles say every 3k then that should be fine as there will not be much difference in the tread depths and therefor oversteer will be much less!
hope this helps!