every truck rear diff i build gets driven 100 miles before i release it to the customer, but that is cause the ones we build are for idiots that pull 20k behind an F350.

standard gears in a car dont need any break in as far as i am concerned, just the test drive to make sure that the gears are quiet and such. the main thing is to make several figure 8's in the parking lot, then come back in and reset the fluid level. i have had almost half a gallon have to be added after doing that (it sends the fluid to the outboard ends, namely the axle bearings and tube)

like anything else, break it in like you are going to drive it.