You ask 10 people, you'll get 10 different answers. The rings are going to either seat or not seat within the first 15 - 30 seconds the car runs. I don't personally believe an engine break-in is necessary, however, with a new hydraulic roller cam I would spend a good 20 minutes breaking that in. Then again, many people have commented how their newer Mustangs have felt like someone has flipped a switch to a more aggressive tune after 1500 miles, so maybe Ford is on to something...?