I would NEVER lap a CPU. I have never seen a heatsink so bad that it would require lapping. (though if I saw one, I wouldn't hesitate to lap it.) Heatsinks I've seen have always been rather flat, I doubt rubbing out the machine grooves makes that much difference.
Speaking of which, even after lapping, you can really only expect a 1-3c degree difference. You can get this difference simply by removing the stock thermal tape and using quality TIM. Would probably cost less then buying sandpaper, time to sand it, and the lower quality TIM. Not to mention there is no risk of damaging anything.
For me, I view it as a waste of time. Unless you actually buy a heatsink where the bottom is as curved as Aunt Martha's glasses.