ORIGINAL: rcnitrohead
Idling will not put any stress on the rod, but rather taper the piston size at the improper temperature. So when break in is finished and you lean out, you experience "blow-by" and a lack of compression at proper operating temps[8D]
If your engine has a lot of pinch and you don't preheat the engine and maintain the temps over 200 degrees F, you can damage the con-rod.