Give it at least one tank, preferably more before you start to lean it out. Break in is basically what it sounds like - it's making sure all your parts fit together like they should, and putting that right where there's slight (thousands of inches) manufacturing differences...

You'll notice that TDC is ***VERY*** tight in a brand new engine, to the point where if the engine's stopped there you have great difficulty in getting it moving again (spoken from experience). Breaking starts to loosen things up a bit.

The parts in your engine actually contract when they're heated, so running the engine at high temps straight away isn't going to break it in properly (that's how your piston gets stuck - make sure you're not at TDC when you let the engine cool!).

If I've made no sense, then ignore me. It's 7:56AM and I've still not been able to sleep :P