Break-ins are always full of myths and legends. With the industrial standardization there is today, it's very rare for parts not to fit correctly as it used to happen, which is why powertrain warranties are getting longer. It's true, though, that a lot of parts have protective coatings (tires, brakes, bearings) and will not perform their best until they're lightly used or warmed up. This is what I do and recommend for the break-in period:
- Do not drive too fast for the street you're in (I'm not into the "speed kills" bullshit, but you won't be able to do a proper emergency stop until your tires/brakes had a few kms of use).
- Do not redline the engine until it reaches the optimum operation temperature.
- Do not accelerate hard or reach high speeds until the transmission and differential oils are warm and have lubed the gears properly - be advised that they take longer to warm up than the engine and the car needs to be moving for proper lubrication.
- If possible, wait a couple of minutes after start-up for the engine oil to circulate and lubricate heads, pistons and, in OHC engines, cams.
- Avoid using the stereo - keep listening to the mechanical components. A strange sound might warn you of defective parts before they become damaged.