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.
The only thing I lapped was an old (7 years ago) Thermaltake HS, it was convex. (kept moving on me, and had terrible contact) point of advice never lap the CPU, unless you don't want a factory replacement when it dies! (and it will) You also have to worry about sanding too much off (breaking through the shroud) which is really bad. But a good quality thermal paste (ie.. AS5, or ceramique, others can name more) will do more than the amount of time and effort it takes to lap the HS, and CPU.